My Little Pony: Friendship Is Magic/WMG/Royalty

Guesses about the Royal Pony Sisters and possible kin. If you have a guess about something else, then

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Celestia is a Bitch in Sheep's Clothing
She invited the ponies to the Gala with the intention of humiliating themselves for her own amusement.
 * She used the Elements of Harmony to banish her sister to the moon, when she could have used them to heal her, like Twilight did.
 * She also kept Luna locked in a basement or something during most of Season 1.
 * She said outright that she didn't know what she was doing. This troper is under the impression that the power of the Elements can only be successfully wielded by six individuals with a strong connection to their traits - even a god can't harness their power alone.
 * She allowed the people to believe she is the princess by divine right, when Ponyville had been previously ruled by humans (i.e. Megan's ancestors, Scorpan).
 * Megan isn't even from the same universe as the ponies.
 * While I am not sure if it was made cannon, it was originally intended for to Megan be the decesendant of Pony World's former rulers. Whether or not it was, Megan herself was more or less the ruler of the Ponies.
 * Whatever happened to Megan?
 * On a related note, the only types of ponies in her kingdom are the ones that somewhat resemble her (the Earth ponies, the pegusi, and unicorns). What the hell happened to the Flutter ponies or the Sea ponies? Celestia must be a fan of ethnic cleansing...
 * The Flutter Ponies were isolationists, they're probably living happily of Flutter Valley.
 * True, but Twilight once said that only unicorns are capable of magic, despite the Flutter ponies having powers. This could mean Flutter ponies don't exist, or that they were simply horribly massacred by Celestia.
 * From what I recall the Flutter Ponies always used their powers to counter other beings magic. In other words they didn't have magical powers, but anti-magical powers.
 * Which makes them all the more likely to be destroyed by Celestia.
 * Who's to say that Twilight even knows Flutter Ponies exist? Sure, she's smart, but if they're isolated away from the rest of pony society, everypony might just be unaware of them.
 * Stop denying the obvious! Where do you think the ponies' marshmallows came from?
 * Rarity.
 * Finally, even inviting Twilight to live in Ponyville was a gambit. Ever since Twilight arrived Pinkie and Fluttershy have gone bat-shit insane, Rarity has gotten more whiny, Applejack has become less honest, and Rainbow Dash has become less loyal, and a lesbian for some reason. Now, they are losing the control over their elements, and Celestia has no reason to fear the elements being used on her.
 * It gets worse. I am sure Celestia is also the reason "Everypony Loves Derpy" was canned after two episodes. That jealous bitch wants to make sure she is the most loved creature in all of Equestria!

Princess Celestia and Lady Rainicorn are distantly related.
Both have royal titles, both seem to be made of rainbows to some extent. Maybe they're distant cousins. Another possible theory is Lady Rainicorn is Celestia's great-great-great etc. grand-daughter. The time-span is so long that Rainicorn's kind have lost their wings, but can still "fly" by manipulating light. Yeah, I don't have a whole lot of evidence behind this theory.
 * Alternatively, Equestria is in the Crystal Dimension.

Princess Celestia used to be Princess Daymare
Before Nightmare turned crazy, while she still ruled the nighttime alone, her sister Daymare ruled the day. After Nightmare was sealed in the moon, Daymare changed her name to fit with her new responsibility towards the entire celestial sphere, instead of just the daytime.
 * Since her sister is named Princess Luna, and only took the name Nightmare Moon after her Face Heel Turn, Celestia was probably named Princess Sol-something (since just Princess Sol would sound silly).
 * Princess Solara?
 * Soleil?
 * Soleanna? Oh god no...
 * But "Celestia" is already a reference to the sun. Hell, it's one letter away from simply meaning "Of the sun."
 * As mentioned below, Celestia comes from the Latin word "caelestis", which is an adjective that means "heavenly" or "of the sky" (Sky/Heaven = caelum in Latin); it's very commonly used of Graeco-Roman divinities for obvious reasons. So Celestia's name is very appropriate given her realm of influence, benevolence, beauty and semi-divinity.
 * Since the cartoon already used Spanish words, I'm guessing Celestia's original name was "MareSol".
 * Luna is not necessarily Spanish; it comes from Latin, actually, and probably Celestia's is also inspired on such language (it's not Spanish, that's a given: "celeste" is "sky blue (color)", and "celestial" is "heavenly; from the sky", neither is sun)
 * I was talking about the "Cute-ceañera"
 * How about "Princess Helia?"
 * Maybe, being the elder sister, Celestia was always intended to be ruler of the entire night sky, aka the celestial sphere. When Luna came along (unexpectedly?) she was made ruler of the night sky, but Celestia kept the name.

Princess Celestia is grooming Twilight Sparkle as her replacement.
And, by proxy, will turn her into a flying unicorn much like herself when she proves worthy.
 * Similar idea? I think the clue is in her name: Twilight is the phase in between day and night. Twilight Sparkle is intended to serve as a bridge between day and night - between Celestia and Luna.
 * Expanding on this idea: depending on how Twilight turns out, Celestia may have plans to form a whole pantheon of six beings of equal power to her own. Because as both pilots show us, not having a six-pony fellowship of their own to unlock the power of the Elements comes back to bite Princesses Luna and Celestia in the ass every time there's an apocalypse on the horizon. We are currently at the fourth time this has been said to have happened.

Celestia plans to turn Twilight Sparkle into another winged unicorn.
By deliberately invoking Power Gives You Wings. This ties into the series being Merchandise-Driven; if it sticks, fans would want/feel they have to buy a new, winged Twilight Sparkle to match the one on the show. Also, this could also provide a bit of Body Horror for Twilight and require an adjustment period, with Rainbow Dash and Fluttershy helping her learn how to fly and fearing being socially outcast based on her strange new appearance. (This being Ponyville, such fears would be largely unfounded.)
 * That would leave us with three winged unicorns - one in charge of the day, one in charge of the night, and Twilight in charge of the time between.
 * Or maybe stars? --Oh Shit, what does that mean for the stars that freed NMM?
 * One of the animators of the show has her own take.
 * Well, her name is Twilight Sparkle. Some significance maybe?
 * Magical daughter of Luna and Celestia? (Twilight is halfway between Day and Night after all)
 * Now please imagine an immortal Twilight Sparkle talking to a fully grown Spike. Isn't it both heartwarming... and terrifying?
 * Also think about the fact that spike is going to live a very very long time, Celetstia's pet is a phoenix, something that lives forever or a very long time, the egg test was to see who was next in line for either the throne or ascension into becoming an Alicorn.

The Summer Sun Celebration is Luna's birthday
Longest night of the year were everyone stays up, and the main reason Luna went evil-ish was everyone sleeping through the night.
 * Longest DAY of the year, and it's called the summer sun festival, definitely a festival in honor of God-queen Celestia. Luna's main festival is probably Christmas, the actual longest night of the year. Some winter holidays definitely happen, and are quote "Awesome holidays".
 * Why would you have your birthday party on the day of the year you have the most work to do?
 * Christmas is December 25th, the longest night of the year is December 21st.
 * Let's just assume that the ponies, which are presumably not Christian, run some sort of solstice celebration on the appropriate day. And it is apparently awesome.

Celestia rules Equestria with an iron hoof.
Through careful application of her own skilled magic, which lets her manipulate the earth and heavens, she has created a rather dystopian monarchy. Under Celestia's rule, a caste system has formed; a matriarchal hierarchy has been put into place, where the magical unicorns outrank the flying pegasi, and everybody is above the average, non-magical ponies. Celestia herself is above everything: A goddess incarnate, she appears as a winged unicorn, a completely unique creature beyond the powers of nearly everything around her, save for the Elements of Harmony, a strange magic beyond her control. In this story, Celestia has groomed Twilight, a talented young bookworm, as her "student," and discreetly tasked her with analysis of the power of friendship magic, something Celestia has never controlled. At the end of every episode, a loyal, naive Twilight writes to Celestia with her findings on the magic of friendship. (It should be noted that this troper is not a follower of the older series; if this is a restatement of some other canon description of the caste system and power-hungry princess, please leave a comment.)
 * The escape of Nightmare Moon was just an elaborate ploy - she either never existed to begin with, or if Celestia was benign back then and the story is actually true, is still imprisoned on the Moon. Celestia wanted to exploit the Elements of Harmony, but genuine friendship was required to power them up from their inert state, something which she either never understood or has forsaken long ago. So she faked the escape of Nightmare Moon, and put the ponies through a series of carefully planned obstacles to bring out their respective elements. When the ponies defeated the illusionary Nightmare Moon, she took the Elements with her for "safekeeping". She keeps putting up a good face for the cast because their friendship might be needed again should the Elements go inert, and so they aren't used against her before she figures out how to seize their power.
 * All the other ponies think that the legend of Luna was just a myth. This means that Celestia has been lying about what happened for 1000 years, denying that her sister ever really existed or that she was ever not in complete control of the heavens. Twilight only finds out about this by poking around in the royal library, after which she is tricked into exiling herself. The entire pony society is designed as a "Brave New World" structure where nature is artificial and everypony is kept busy, preventing political unrest. The one place we know of that is outside of her control is called 'Everfree' Forest for goodness sakes!

In the pilot, Princess Celestia wrote her letter to Twilight in advance.
She responded very quickly, and the response is a little vague, never mentioning Nightmare Moon at all.
 * In the next episode after the pilot, Ticket Master, one of the ponies actually says "that was fast!" when Princess Celestia immediately responded to Twilight's mail with a letter containing extra tickets.
 * In Ticket Master, it's rather obvious that Celestia deliberately sent only two tickets as a test for Twilight.
 * Go listen to how the letter was written again. It's actually rather obvious that Celestia didn't personally send the tickets; it was a form letter.

Twilight is Celestia's daughter... kind of.
Long before the rebellion of princess Luna, and possibly before Celestia became so powerful, Celestia had a daughter who was killed in some sort of magic related accident. A dragon attempted to save her, but received mortal wounds in the process. When Celestia finally reached the two, they were already doomed. In a stunning display of arcane magic, Celestia cast their souls to be reincarnated some day. The spell could only guarantee reincarnation in some way. the time until the reincarnation, the location, and the heritage were all uncontrollable, though because she set both the dragon and her daughter to be reincarnated in the same spell, the two were bound by destiny in their next life. Fast forward at least a thousand years, and Twilight turns up in an orphanage, And a dragon egg turns up at the royal palace in Canterlot. Spike, as a dragon with strange properties, remembers his past life and retains most of his knowledge, including the events of his and his destiny to guard Twilight, making him surprisingly intelligent for an infant dragon. Twilight is taken under the guardianship of Celestia in private, where either directly as an adoptive mother or indirectly as a teacher and mentor, she is raised by Celestia.
 * Taken directly from a fanfic.
 * Well, the whole "Twlight is an orphan" part's been Jossed now. Doesn't leave out the reincarnation part, though.
 * Her parents might have been committed to a mental health hospital after the entrance exam, which could technically make her an orphan.
 * It still doesn't rule out the possibility Twilight being descended from Celestia somehow (or perhaps being the descendant of the original wielders of the Elements of Harmony).
 * There's always the possibility that Twilight Sparkle was adopted, or that her parents are, in fact, an aunt and uncle. Alternatively, she's a descendant of Luna, rather than Celestia. The two of them seem more similar than Twilight and Celestia, though a millennium of genetic drift may not make that last part a definitive reason.
 * ...Link please?

Twilight Sparkle is Princess Celestia's avatar
Because the Princess can't use the Elements of Harmony anymore she imbued a newborn filly with some of her essence in time for Nightmare Moon's release.
 * What her friends are depends on whether Celestia had help the first time.
 * If she did have help they're reincarnations of Celestia's group of friends.
 * If not, they are also her avatars. She split her essence into portions corresponding to the six Elements. This also explains how they all got their cutie marks on the same day, triggered by the same event.

Princess Celestia is actually Amaterasu in a horse incarnation
Princess C controls the sun, and her servants directly manipulate the plants, weather, animals, and seasons. In Okami, Amaterasu was shown to be able to do most if not all these things. They are also both female, white with physical characteristic unlike regular versions of their species, and act as rulers, either as goddess or princess. They even look kind of alike, if you replace Amaterasu's fire with a rainbow, as seen here and here
 * Horses are also sacred to Amaterasu. I mean the real mythological one, not the video game one.
 * There is also Kazegami, the god of wind, who is a part of game!Amaterasu and is a horse.
 * i think she incanated by her own

Celestia is properly Princeps, not Princess
Equestria has overtones of rome, at least the royal palace does. Celestia rules not because of her birthright, but because she is the first among equals. She's called a princess in the show because kids wouldn't know what a princeps is.
 * Since she is a she, she would properly be a principissa, which is the feminine form of princeps. Also, nothing about the title of princeps or principissa, which are after all simply Latin for prince and princess, necessarily implies that the one so entitled is first among equals. It is true that Octavian used the term that way, but the fact is that long before that time, princeps meant leader, chieftain, or ruler, or, in other words, prince.

Princess Celestia plans on turning Twilight Sparkle into a Pegasus Unicorn by taking over her body
The reason why the Princess has lived so long is that she takes over her student's body once every thousand years. The whole Nightmare Moon story is a setup to find a proper vessel and the Elements of Harmony is the final test of the worthiness of that vessel. Luna is in on because being sealed up in the moon means she doesn't have to parasitically take over another pony's body.
 * Why does this remind me of Dragon Age Origins? =)
 * Perhaps Luna is so young because she did the same before she was sealed in the moon, and shortly after Celestia takes Twilight's body, she would be sealed in the sun. The seal wasn't actually a punishment, but rather a cosmic arrangement - right after the Princess takes a body, they are heroically sealed away by their sibling for a long time so all the subjects will forget. Nightmare Moon was just a temporary form of Power Incontinence after being sealed for so long.

Princess Celestia went through a tyrannical phase, but grew out of it.
She's had over a thousand years to grow and change. The trauma of having to fight her own sister and ultimately sealing her away, meaning she also had to shoulder responsibility for manipulating the moon on top of the sun, reallllly didn't help. On top of that, if she had friends fighting alongside her to help wield the other Elements of Harmony, she had to watch them grow old and die while she kept on living, continuing her duties with no end in sight. So she found ways to... cope. Some healthier than others.

However, she eventually realized she had become distressingly similar to Nightmare Moon. Despite presenting a pretty and idealistic image to her subjects, inwardly she had grown cold and detached, viewing them as mere subjects where once she would have counted some of them as friends. She also came to understand the sort of bitter loneliness and frustration that led to her sister's betrayal in the first place. Gradually, Celestia grew from wearing the queenly mask into an honestly benevolent ruler. With Luna's return, she hopes to reconnect with her as well and hopefully keep her from falling back into the same pitfalls and temptations.
 * Given Luna's "traditional" behavior in "Luna Eclipsed", it could well be that Celestia was a tyrannical, or at least strict, ruler Back in the Day. Luna was one of the few people she could open up to in these circumstances and, after the Nightmare Moon conflict, the above occurred.

The way everyone acted toward Celestia in "A Bird in the Hoof" is a veiled parody of how the fandom acts towards Lauren Faust.
With perhaps some parody of "Celestia is a tyrant" WMG's thrown in.

Princess Celestia has a Bene Gesserit style breeding program that produced Twilight Sparkle
She bred her and Luna's bloodlines over the past millennium to produce a pony that could defeat Nightmare and/or become her successor. Twilight's friends could have been bred as well, probably descendants of Celestia's mortal friends.
 * Did she really do this just to defeat Nightmare? Or is there some even greater threat yet to come? That said, I like this theory. It opens up some very interesting possibilities for Crossover Fanfiction. I for one like to consider a combination of this guess with my guessing above that Equestria is a plane of Dominia.
 * Just to defeat Nightmare? Keep in mind that not only is Princess Luna her sister, without her Celestia has to control night and day and be on duty all the time. Even an evil Princess Celestia would view finally having some time off after a thousand years as a very high priority.
 * Maybe she's supposed to be the pony Kwisatz Haderach too?
 * A corollary of this is that all of the mane cast were specifically bred over several generations specifically for the right traits. That's how Celestia knew that Twilight would find friends perfectly suited for the other Elements of Harmony in Ponyville- that's where the other five bloodlines were living.
 * Social engineering was also probably involved- for instance, ensuring that a pony genetically predisposed towards maximum happiness was reared in such a dreary place that the inevitable personality backlash would lead towards subconscious compensation by exaggerated fun-focus, resulting in Pinkie Pie.
 * This theory gains credence with the appearance of Twilight's Brother, who is powerful enough to protect all of Canterlot by himself.

Celestia is slowly overtaking Twilight
Remember the line from the opening: I have always wondered what friendship could be. This does not describe twilight, because as we saw in the pilot, she was quite dismissive of the concept. But it's still said with her voice, so how could this be? Easy; Celestia is supplanting Twilight, so she can properly understand friendship.
 * Wait, so Celestia is possessing Twilight?
 * That, or just living through her vicariously. Celestia's job, age, and public image (see above WMGs) have kept her from making friends. Celestia sent Twilight to discover "the magic of friendship" because it's something that the Princess legitimately has not been able to learn for herself. Celestia didn't even have the "my sister is my best friend" option.
 * Celestia has much longer legs than Twilight, and she has wings. If she wanted to overtake her, it would happen quickly.

When she had the Elements of Harmony, Celestia's element was Laughter.
Besides 'Trollestia,' there's the fact she does laugh a lot, the way she makes fun of her subjects being overly adoring and attentive with the tea, the fact her pet/friend Philomena is such a troll... However, a thousand years without her sister for company after her friends died, the strain of looking after a kingdom 24/7 instead of half the time and having to control both day and night have worn on her. Now that her sister is back and Twilight Sparkle's adventures seem to be making her feel young again, she will start acting more like Pinkie Pie as she returns to her old self.
 * It's possible that Luna was one of the wielders of the Elements along with Celestia. If so, her element was Generosity: the feeling that her gift was rejected was what left her open to Nightmare Moon.
 * If Luna was a carrier of one of the elements of harmony, then how was it possible for the elements to be used? Because Luna's elements was Loyalty, and in the end, Luna was able to break through Nightmare Moon just long enough to use the elements of harmony against herself, preferring to banish herself than betray Equestria.
 * This is interesting. I had assumed that Celestia's element was Magic. But this is indeed not as self-evident as it may seem.
 * I'd bet Celestia's element was Magic, specially since Magic = Friendship here, and it's clear Celestia values that a lot considering how much she wants Twilight to learn about it. As for Laughter, couldn't Philomena be the holder of it? Also, I'd agree that Nightmare Moon's element might have been Loyalty.
 * Another alternative: Maybe Celestia combines all elements, but that wasn't always the case. Think of it: It is true that she embodies Laughter, but she also embodies Loyalty, Honesty, Kindness and Generosity. Maybe if someone who is a Wielder of an Element dies, his or her personality doesn't just vanish, but somehow "merges" with the personalities of her friends. In the end, when only one body remains, we have the "souls" of all wielders united as one person, in one body, who then ascends to the semi-Godhood of a Pegasus Unicorn form. This would also prevent the Unfortunate Implications of Who Wants to Live Forever? and of Celestia having outlived all her friends: she hasn't, they're always present within her, as a part of her. Also, one might consider the possibility that Celestia and her friends never had the sixth element. It was hidden as a spark within Twilight after all. So, in her, the personalities of only five wielders are united. If this happens to the wielders of all six elements, the result will be a literal Goddess. (Or something like that.)
 * Or perhaps she does embody all six and is attempting to create another goddess, as in the Mane cast are being set up so they'll merge.
 * Has anyone written a fanfic like that? Because I'm getting the urge to write a Death Fic with a "happy" ending for some reason.
 * There's a pair of stories where an Evil!Twilight, at Evil!Celestia's urging, attempts to kill off all the other bearers so she can take over their elements, but I've never seen one that took a "happy" spin on the concept.
 * Funny, I always got the idea that Celestia could use all the Elements of Harmony singlehandely, if not with the same might, thanks to be a god. And that's why she could only seal Nightmare Moon in to the moon instead of completely reforming her sister.
 * I always seem to envision that when they were the age of the Cutie Mark Crusaders, Celestia would sometimes prank the guards (i.e. buckets of water), while Luna was always shy and needed a helping hoof from her sister. I personally think that's a fitting backstory.

The sun was not Celestia's original cutie mark.
When the mane six get their magical jewelry during the Elements of Harmony sequence, the gems match their cutie marks. This may mean that Celestia's original cutie mark, before some Noodle Incident we'll never see, was that purple diamond set into her armor/harness and crown.
 * The founders of Equestria must have had the ability to change their cutie marks at will, using them as a quick way to show what their jobs or interests were without having to say anything. Centuries of genetic drift stunted the ability to the point where the latest generations can't control it at all and have never heard of anyone who could.
 * In addition to that, Celestia might not be her original name. Considering Luna=Moon, wouldn't something like Solaris make more sense? She could have took the name Celestia after taking over her sister's responsibilities to reflect her new position as in charge of the two celestial bodies.
 * I have to disagree with my fellow tropers on this one. I believe that Celestia's Cutie Mark was always the sun. The Elements seem to have the ability to mold themselves to the personality of the pony using them. Remember when the elements were rocks? After a millennium of not being used, by either pony or Alicorn, they reverted back into their impotent or power-saving mode waiting for the next pony or ponies that fitted the qualities of the elements. The purple diamonds on her armor and crown denote her royal status, as purple is traditionally the colour of royalty.

Celestia doesn't want to be a Princess
"Bird in the Hoof" seemed to be invoking a bit of Stop Worshipping Me!. Maybe she just happened to be an immortal unicorn-pegasus who could control the sun and unintentionally attracted a throng of worshippers who insisted she rule them (and she chose the title "Princess" because she didn't like "God-Empress").
 * Which could tie into the ones suggesting that she's grooming Twilight to be her successor.
 * It also means Luna had little reason to be jealous.
 * She was jealous of the attention the day gets, not of Celestia herself.
 * She doesn't seem to stand on protocol very heavily, does she? That said, she did have guards on the door, and doesn't do much to discourage formalities in other situations. More likely, the brunch was intended to be a relaxed event where Celestia could meet and get to know her star student's friends and acquaintences. Why else would she be in Ponyville in the first place?
 * Well, the brunch may have been an apology for having to cancel her appearance due to the parasprite invasion in Phillydelphia.
 * She pretty much arranged for Twilight's friends to trash the stuffy Grand Galloping Gala.
 * She could be simply just tired of it. Maybe after a thousand years of the same genuflection and adoration, not to mention whatever adoration she had before, it had gone tedious and boring to the point of insanity. And she had found in Twlights and her friends scapegoats, from her letters where she lives vicariously to the fac that since they are the Elemental wearers, allow her a lot of political protection and support; hence why they could destroy the Grand Galloping Gala without repercussion.

Celestia was hiding her depression for centuries until being reunited with Luna after so long.
I can see it: right before using the Elements of Harmony to seal Nightmare Moon away, Celestia sheds a Single Tear, knowing that her sister Luna would have to be sealed away. After learning that the 6 fillies that would bear the Elements have been born, she sees this as a Hope Spot and decides to teach Twilight Sparkle, the future bearer of Magic, which helped her earn her Cutie Mark. And after reuniting with Luna after 1,000 years, Celestia was no longer living with the pain of being separated from her sister.

Celestia invented the Sonic Rainboom.
Only two Ponies have rainbow coloured hair and Dash said the move was a legend before she proved it could be done. Given Celestia's age (and the apparent inability of ponies to record history) it is possible that she came up with the move when she was young and impulsive but stopped using it, perhaps due to the trauma of her sister going nuts, causing it to fall into myth.
 * Celestia's version probably wasn't as impressive as Dash's version would be, since Celestia's hair only has four colors while Dash's has the full six.

In "Swarm of the Century", Princess Celestia knew exactly what was going on.
It seems unlikely that Equestria's ruler wouldn't know what a parasprite is, given how dangerous those creatures can be. And she probably saw that Ponyville was in trouble from the air. When she saw polka-playing Pinkie Pie's parasprite parade go by, she put two and two together and, figuring they had everything in hand, used having to go to Fillydelphia to let the ponies save face and get to work rebuilding the town.

By sending Twilight to Ponyville to get some friends, Celestia also tries to find new friends herself, "by proxy" if you will.
The real reason why Celestia cannot just find new friends herself is that nearly all other ponies behave completely subservient to her, as seen when Mr. and Mrs. Cake kept refilling her tea cup in "Bird in the Hoof", and you cannot build a friendship on that. So, by sending her "faithful student" to find some friends, she hopes that those friends might eventually also become friends to herself.

Princess Celestia planned everything
She's trying to teach Twilight (and maybe her friends too) everything about friendship for one or more of the previously mentioned reasons and the entire season so far has gone Exactly as planned.

Princess Celestia had her own party crashed so she could finally be casual with Twilight's friends
Throughout the season she has attempted to meet ponyville casually, attempting to be treated like a normal citizen and trying to really get to know Twilight's friends. Every time beforehand, her attempts were ruined by either circumstance or citizens taking her "casual" visit far too seriously, making her nigh unapproachable to them. After Twilight and her friends regrouped fully shamed at the doughnut shop, Celestia sincerely thanked them for screwing around with the aristocracy and in general making the night fun for once, making her appear as less a goddess or royalty, and more as just another pony. After the credits rolled, Celestia spent hours with the mane cast in the shop as a peer in their eyes and in the process she made some of the first real friends after hundreds of years of the aristocracy sucking up to her in canterlot.
 * This makes a lot of sense when one realizes the one being who could possibly be a friend to her even in her position of power was sealed away for 1000 years.

Relating to the above, Celestia will be more candid with the mane cast in season 2.
With the mane cast being friends on a personal level, and Luna able to fill in for her days off, Celestia will occasionally sneak out of Canterlot. Either disguised for a day with her friends, or just quickly popping into the library undisguised for a short visit. Though she will be more personable and commonly occurring, she will still have the same effect on the viewer as she did in season one because of how understated her appearances will be when casually showing up.

Celestia trained all those animals in the Gala to specifically avoid Fluttershy and make her mad.
Since Celestia has proven herself to be a grade-A Troll throughout the series, I wouldn't put it past her to do that.
 * After the Philomena incident, I wouldn't be the least surprised. And she did want Twilight's friends to liven up the Gala.

In "The Best Night Ever", Celestia had another reason for going to the donut shop: saving the ponies.
Shortly after the Gala was ruined, warrants were put out for the arrest of the main cast. (Or at least some of them: Rarity, Rainbow Dash, and Fluttershy were probably the ones that caused the most damage.) While the ponies were telling Spike about what had happened, the police (or city guards, same difference) were amassing outside the shop and ready to bust in and arrest them. Celestia intervened, gave the cast a royal pardon, and assured the police that she would deal with the situation personally.
 * Incidentally, when the warrants were first put out, Celestia was too busy handling the fallout of the Gala to deal with it or even learn about them at first.
 * It seems her plan to use them to energize the Gala went horribly right.
 * Princess Celestia has probably issued several pardons for the chaos the main cast has caused over the series, as she needs them to continue serving as her secret commandos.
 * None of this seems to make much sense. After all, what would they be charged with? Rainbow Dash just tried to catch a falling statue; the fact that it didn't work out does not make her a criminal. Likewise, all Rarity did was berate her loutish date and shake out her mane. Even Fluttershy only burst through the door and screamed. I suppose you might charge her with malicious mischief or some other misdemeanor of that nature, but worst case scenario, she'd have to pay a fine or maybe do some community service. But she didn't really commit any crime. Also, Celestia is an absolute monarch; no one is going to arrest her prize pupil or said pupil's friends without Celestia's direct authorization. Maybe if they caught one of them red-hoofed committing a major felony, although even then they might want to get Celestia's permission first.
 * Twilight's older brother being Captain of the Guard is just more reinforcement for the premise that nopony is going to be attempting to arrest Twilight Sparkle for anything short of attempted murder.

Princess Celestia is secretly a Kamen Rider.
Well, she has a sibling who reappeared with a name relating to darkness and the moon. And she's associated with the sun...

Princess Celestia and Megan are Alternate Universe counterparts to each other.
It was previously suggested that Megan and Celestia were the same person/pony, but was Jossed since Celestia and Luna have no siblings. Furthermore, since G1 Twilight is Twlight Sparkle's mom, she'd have to be around the same age as Celestia and Luna. However, having them be the same pony/person from different universes would resolve these issues.

Princess Celestia really did raise Spike after Twilight hatched his egg.
Spike and Twilight had only been paired together for a year or two prior to the pilot because Princess Celestia thought he would have a good influence on her and encourage her to lighten up. "Official Canterlot Business" is really code for "Young man, come home and visit your mother."

Celestia arraigned a marriage between Twilight Sparkle and Prince Blueblood
Neither pony knows it yet, but Celestia intends for the two of them to marry and produce a foal of noble birth (Blueblood's part) and immense magical powers (Twilight's part). Then this foal will become the next ruler of Equestria.
 * Oh God, no...
 * Next ruler? But Celestia and Luna are immortal, to all appearances. Why would they need a successor?
 * So they can retire and not have to deal with the tremendous stress of ruling AND fulfilling cosmic duties?

Celestia was a bigger prankster than Pinkie Pie as a filly.
There's a reason for the nickname Trollestria after all. And I like to imagine that Celestia has only mellowed out somewhat with her age, and as a young energetic filly? She must have been absolutely brutal with her pranks.
 * For maximum effect, imagine an episode where Pinkie and filly!Celestia try to out-prank one-another.
 * When she was a filly? She may be a bigger prankster now. Seriously, what has been Pinkie's biggest prank to date? Having Rainbow Dash hit Spike with a lightning bolt? Celestia deliberately wrecked the biggest event of Equestrian high society.

Celestia is a fusion of the last group to wield Elements of Harmony
Which is why she's a Unicorn Pegasus
 * There are six Elements, and Word of God states that Celestia and Luna are a combanation of all three types of ponies, meaning that three of the main six will fuse into one alicorn, while the other three will fuse into another alicorn, explaining the existence of the two princesses.
 * yes, yes!! Just like Asimov's Novel The God Themselves! So let's see:
 * My bet is TS will be merged/fusioned with RD and PP.
 * Several other WMG in this page are also valid: Celestia is grooming the Mane 6 to be her substitutes after fusion, etc.

Celestia was planning to use Twilight and her friends ever since the Sonic Rainboom.
The day before the Summer Sun Celebration seems a bit late to be implementing a plan to save the world. When Dash's first Sonic Rainboom indicated a connection between the six friends, Celestia detected the connection (as she is wise in the ways of the Magic of Frienship), and knew that this was her best bet for stopping Nightmare Moon and rescuing her sister. That was her main reason for taking on Twilight as her primary student (in addition to her general magical aptitude), and she also started monitoring the others at this point - she may have been responsible for Pinky moving to Ponyville, for example. She made sure that those five would be the people Twilight met on her first tour of Ponyville, and the rest proceeded as planned.

== Celestia not only told Ditzy/Derpy to go west and north to get the southbound birds so as to add her brand of 'merriment' to the Winter Round-up, she also paid her to drop all that stuff on Twilight Sparkle for cheap laughs. == Even a master-mind genius prankster like Celestia can't be everywhere spreading mirth. That means that she'd need a stooge or five hanging around to help mess with her subjects' heads; my guess is that someone with strabismus is being punished for some mild misdemeanor with community disservice.

Derpy/Ditzy is Celestia in disguise.
Why else do we see the adult version of her hanging around when Twilight got her cutie mark? It could well be that Celestia does the occasional bit of covert surveillance on the Bearers of the Elements in her original form: a bluish-gray pegasus pony with a blond mane and amber eyes. The derped eyes and crazy antics are just her way of having fun.

Celestia is grooming Twilight for a high-ranking position at court.
Related to the WMG above that Celestia is grooming Twilight to be her successor, but different. After all, why would an immortal Goddess-Princess need a successor? Besides which, if Celestia has no foals of her own, her legal heir would be her younger sister, Luna. No, she's training Twilight to someday hold an important political position at court: Hoof of the Princess. Think about it: at the premiere social event of Equestrian society, Celestia kept Twilight at her side all night while all the other guests made their obeisances. That's basically a declaration to the entire aristocracy of Equestria that they should get used to the idea of bowing to Twilight Sparkle. Also, Celestia didn't just send Twilight out to make friends just so that Twilight would be happier; she wanted Twilight to learn how to recruit political allies. And why Ponyville? It's a place where the previously sheltered Twilight could learn more about the lives of pony commoners. And what has Twilight been learning? How to manage and govern ponies. Just look at the events of "Winter Wrap-Up".

Most of Equestria is descended from Celestia
Inspired by this side-story to Progress.
 * Though really she is royalty, a Physical God, and a thousand years old, it just makes sense.

Part of the reason Celestia arranged the Mane Cast to attend the Gala was to have geuninely good people there
The only genuinely good people we see at the entire thing are the Wonderbolts, everyone else seemed to really be a high and mighty Jerkass (especially Prince Blueblood). She's been to the Gala over and over again for centuries and probably saw the exact same thing over and over again. Not only did she want to spice things up a little, she wanted to make it nicer by inviting people she knew were genuinely good.
 * I beg to disagree, other than prince Blueblood, none of the other guests were outright disrespectful to the cast unless they were interrupted or disturbed by them in some way. Formal parties do have rules and etiquette, things that none of the girls have knowledge of. All of them, with the exception of Rarity and Twilight, caused trouble for the guests so how did you expect them to act? I know if I had seen Pinkiepie's antics I would have told her off too.

Discord is Celestia's old Boyfriend.
Just getting this out there before the Season 2 Premier. Nine more days!
 * Chances are they'll get shipped together anyways.
 * Now that we know what Discord looks like, if this is true, Celestia has some VERY strange tastes when it comes to men.
 * "HEY! Tequila was involved, get off me!"
 * "You have no idea how often she says that. It was quite the turn-on."
 * We do know that laughter is Discord's favorite of the Elements of Harmony, and it has been speculated, and not without reason, that that was the element, or one of the elements, wielded by Celestia, so that would fit.

Celestia has been dead since Episode 2
Luna killed Celestia off-screen as revenge and took her place. Discord will be posing as Luna so that nopony will wonder why Luna hasn't appeared since her return. The two will be working together to destroy the main cast so both can conquer the world.

Discord and Celestia used to date.
Their dialogue when Discord was in the stained glass just made me think of it, what with the way he's light and teasing while she is having none of it. I imagine she still holds some lingering attraction to him on a physical level.

Celestia was actually corrupted in Return of Harmony Part 2
She only sent back the Friendship Reports to get rid of what she now considered junk and to make life difficult for Spike. It just so happened that the corrupted Celestia unconsciously gave Twilight what she needed to get her friends back and fix everything.

Princess Celestia didn't send any of the letters back.
Celestia had no idea the mane cast was acting the way they were, so why would she even bother to send those letters back to her? It was all Dischord's doing. His influence caused Spike's mail sending whatever to be chaotic and send the letters back to him. Since this wasn't done normally, it's probably why Spike was in so much pain.

Celestia can perform the Sonic Rainboom
...I got nothing for reasoning, but then again this WMG doesn't need much explanation.

Celestia knows Twilight has some "issues" but doesn't get her a psychologist because she's been treating them herself indirectly
The Season 2 Ep.3 breakdown that Twilight suffered from probably was a symptom of an older issue, hence being triggered by children laughing, and the fixation on the punishment being sent back to kindergarden, and is something Celestia is aware of. The reason Celestia was so mad when she came was because she had hoped support from her friends would of prevented Twilight from relapsing, and because she had hoped having friends would help ween Twilight from being both dependent on her acknowledgement and obsessed with her. She jumped on the opportunity of "Group Friendship Reports" because it would help all of them learn more about and support each other so that situations like the ep.3 breakdown don't happen again.

Celestia is not Luna's sister....
...She is her mother.

Celestia only Trolls for good
Crazy I know but listen to me while she does make things harder, in the end things DO always work out for the best.

Celestia had a pink mane when she was younger.
Since we now see Luna having the flowing energy mane like Celestia has, as opposted to the regular mane in her first appearance, it's possible that Alicorns start off with a regular mane and get an energy one when they get older. In the storybooks recounting old tales involving the princesses, Celestia is always colored pink. It would also explain why the toy version of her is pink as well.

Celestia became a troll when she first tried to connect with her subjects
In Luna Eclipsed, Luna was only really embraced when she embraced their fear of her and started intentionally scaring them, maybe Celestia went through something similar after Luna was banished.
 * Though formality reasserted it's self in between, so Celestia has been trolling to reestablish a laxer attitude with Twilight

Celestia will be corrupted by the same force that turned Luna into Nightmare Moon
She will be far more powerful and dangerous than Luna was, however Luna will figure into the conflict by, via the concepts behind a lunar eclipse, be able to temporarily block or even completely negate Celestia's powers for a long enough period that the Mane Cast can wham her with the Elements of Harmony.

Princess Celestia has other reasons for the Friendship Reports.
It could be, bear with me, it could just be me, but Princess Celestia has been around for more than a millennia, and seen the ponies living by everyday, being friendly and living in harmony. Could it be possible that the reason that the Princess actually asking Twilight to give her the Friendship Reports would be that she's never had any... friends? Think about it: she's been around for a long time, but because of her status as sovereign ruler of Equestria, every other pony out there wouldn't treat her like a friend, but someone outside that circle: an outsider. Not to say that she hasn't made the effort, but since she's royalty, every time she tries, it just doesn't work out properly: she's always treated like the God-Empress she is than the Mare-Next-Door you ask to hang out with you at the doughnut shop.

It could be, in my opinion, that Twilight is essentially there to scout the Magic of Friendship that she's been missing out all these years, and essentially (at least by proxy) make Twilight's friends her friends as well. In my eyes, Celestia really, really wants to make friends and enjoy life as her student Twilight is, but ultimately cannot because of what she is. I think deep inside she wants to buck apples with Applejack; try on new dresses with Rarity; race and tussle with Rainbow Dash; party like it's 1899 with Pinkie Pie and giggle about that studly Big Mac with Fluttershy. Twilight's letters serve a double purpose in this sense: for Twilight, it's a lesson to be learned; for Celestia, it's a little window to a world she knows she can not enter.
 * I like this theory a lot... but wow, poor Celestia.

My theory is that the "friendship reports" were created to be Celestia's eyes and ears all over her realm. I imagine that Celestia has a "protege" in every corner of Equestria periodically sending back intelligence in the form of heartwarming life lessons learned so C. can both get a psychological profile of her subjects and ensure nopony is planning a rebellion.

Celestia's only ever turned Discord to stone
Discord simply mistook ordinary statues Celestia moved into her garden as other enemies she had turned to stone. While ordinary statues would be a more logical thing for Discord to conclude, he might have thought that there's no way Princess Celestia would seriously just toss him in an ordinary garden and put ordinary statues around him. Of course, applying logic to Discord is also pretty invalid since what fun is there in making sense.
 * Or he knew perfectly well that they were ordinary statues and was just trying to, you know, sow discord.

The normally calm and in-control Princess Celestia will suffer a MASSIVE mental breakdown.
She was incredibly serious in the first episode of this season, so maybe it'll show a more expressive side of Celestia. And since all the Mane Six have had a mental breakdown in some form during Season One, there's a possibility that Celestia could wind up having her own meltdown. For whatever reason, it could vary.
 * This could also apply for other emotions that Celestia would express outside of the mental breakdown.
 * Bonus if she shows some genuine emotion for both Twilight and Luna. Cadance, Blueblood, et al not withstanding, Twi and Luna are pretty much the only family she really has.

Someone will prank Princess Celestia by painting her pink.
As a nod to the toys. Also, Celestia loves to do small pranks on others now and then.
 * Considering a white "brony edition" Celestia is supposed to be released in Fall 2012, that pretty much negates this.

They will kill off Celestia by the end of the season.
See Mentor Occupational Hazard.
 * "Kill off" might be too strong a word considering the nature of the show. Put on a Bus, perhaps?
 * Besides, I doubt they would do something like that pretty early-ish in the show. Maybe in The Movie.
 * Jossed, at least for season two.

Celestia is trying to start Instrumentality
She wants to make all ponies one, which is why she is researching friendship.

Celestia was lying about Discord being the former ruler of Equestria, and the real story is much more tragic.
It's clear that she and Discord have a past together, and she expresses a serious and stern attitude about and towards him, however, it's possible that she may have a skeleton in her closet about him. If anything, Discord didn't become the Spirit of Chaos and Disharmony until something happened between him and Celestia. They were probably former friends, and he betrayed her at some point, or he had a crush on her, but she turned him down and called him a "freak". Needless to say, Celestia's rejection broke him. Eventually, he went crazy, and went around manipulating ponies and spreading disharmony and unhappiness with his incredible power, claiming that if he can't be happy, than no one else can. Celestia and Luna then turned him into stone with the Elements of Harmony, which were discovered and used LONG before then, and Celestia made up the story of him ruling Equestria before she and her sister stood up to him after discovering the Elements of Harmony to cover up this shocking secret.

Celestia is the mother of Helios
. Both are white color Alicorns. Both have connection to the sun, Celestia controlling it and Helios being named after the Greek God. Both of them care deeply for a person with a Moon theme and also have connections to Magical Girl.

Fancy Pant's fillyfriend was Celestia in disguise
Note the unusual character design and pink hair.
 * To be more specific, she's a recycled use of Luna's Season 1 model, with the colors of the younger Celestia from the Season One opener. If I may, I'd like to expand this into a complete fanfic concept. The fillyfriend, here-to-after referred to as Fleur deLis, is indeed Celestia in disguise, so she can mingle among the not-royals and learn more about how the government should be run. She also takes note of what kind of prank can be most effective on everypony in the event she feels like going Trollestia later. She hangs around as a Trophy Fillyfriend to Fancy Pants because he's the only pony in Canterlot who is even REMOTELY tolerable to her after 1000 Galas. Fleur deLis makes all those overtly-affectionate and obnoxiously-sexy poses all the time as a cover as a French (Prench?) supermodel. After all, would YOU expect Celestia to do those kind of poses constantly? No, you wouldn't, because that's not Celestia's personality type. The different Cutie Mark can be explained as white paint and a stencil, white paint and artistic skill, white paint and a trusted hoofmaiden with said art ability, or even an orphan with said artistic ability Celestia took in, which would be viewed as "Awww, our princess really does care about everypony." by the general public. Whether Fancy Pants actually KNOWS he's really with the princess is still a matter of debate.

Alternatively, Fleur de Lis is Celestia's bastard daughter.
The father might even be Fancypants, since there is technically nothing indicating that Fleur is his wife/girlfriend. Her affections could be explained by saying she's family. Being such a decent fellow, Fancy gained Celestia's favor in his youth and fathered a daughter by her.

Celestia will be taken over by the same evil force that took over Luna.
Like Luna, she will take an "evil" form and go by something along the lines of "Hellfire Sun", flaming mane and all. Luna will enlist the help of the Mane Cast so they can use the Elements of Harmony and purge the evil from Celestia like they did with Luna.

Celestia was deliberately keeping Twilight friendless.
Celestia knew Twilight was to become the Element of Magic, and had an idea on who else could become the other Elements, and they were in Ponyville. Celestia couldn't afford to let Twilight have other friends (Spike functioning more as family) that could compete with Twilight's affections and ability to recognize the proper Elements.

Celestia couldn't use the Elements of Harmony.
All the Elements require at least two ponies for their expression; they're all the things that make up a good friendship. When Luna was banished to the Moon, Celestia lost her only real friend.

Celestia only gave Twilight one extra ticket because she figured only Rarity wanted to actually go to the Gala.
Let's assume that Twilight wrote some letters between "The Elements of Harmony" and "The Ticket Master," and so Celestia knows the personalities of the Mane cast pretty well. Let's look at how she would expect them to respond: In short, the only ones Celestia actually expected to want to go would be Twilight and Rarity. Too bad she didn;t factor in ulterior motives...
 * Applejack: A rough-and-tumble farm girl who hates the idea of dresses. Too fancy for her.
 * Fluttershy: Terminally introverted. Would no want to deal with so many strangers all at once.
 * Pinkie Pie: Prefers casual parties to formal gatherings. Would get bored with it being so subdued.
 * Rainbow Dash: Atheletic and loud. Would hate just standing around and talking quietly.
 * Rarity: Has aspirations of class and a desire to become a part of high society. Actually wants to go.
 * Spike: Sees it as girly (or so he says). Doesn't want to go.
 * Twilight Sparkle: As the personal apprentice of the Princess, has spent some time in the Royal Court. At the very least, knows ow to handle herself in such a situation.
 * Alternatively, she might have been expecting twilight to being Spike along, since was probably with Twilight for at least some of her time in the royal court and seemed to socialize more in Canterlot than Twilight herself did. Plus there's a buffet. And given the loose timeframe, it's possible that she sent the tickets before she got to know Twilight's friends very well, hence not considering inviting them.

Princess Celestia really needs Twilight Sparkle to study friendship for her...
...because she doesn't quite understand how the Elements of Harmony work. When she first used the Elements thousands of years ago, she had five friends (who have since died, most likely) that made it possible, but none of them knew they were the literal elements. Celestia had no idea why the Elements of Harmony worked, just that they did. She of course, began to study it, and came to understand it to some degree, but by then she needed someone new to wield the elements while she studied them, since she no longer could.

Princess celestia was once the bearer of the Element of Generosity
her necklace and crown have the same diamond shape on them as rarity's element. As the crown looks like twilight's element, but with a diamond instead of a star, this suggests that it was once the main element of an earlier group, much like friendship/magic is the main element of the current group. The diamond suggests that she once had a diamond cutie mark, much like rarity's. and, this is kind of a seperate one, but when she ascended to become the god-princess, the sun overwrote her old cutie mark, as she first and foremost is the bringer of the sun, as opposed to something before that.

Princess Celestia isn't actually a troll/jerk/insert derogatory statement here.
Yeah, yeah, blasphemy. But I'm sick and tired of how everyone treats her like she's Pony-Hitler...
 * Actually, if anything, she's more like King Jigme Wanchuk of Bhutan - more concerned about Gross National Happiness and stuff along those lines. Like Jigme, she's an autocratic ruler (well, like him until he changed Bhutan into a parliamentary democracy) but uses it for good than anything else. Though Canterlot is supposed to obviously mimic the British royalty and aristocracy, I suspect it's daily dealings are more similar to that of Dechencholing Palace than Buckingham's.

Had the Flim-Flam bros. won and kept the popularity of the ponies, Celestia would have vetoed their victory due to the closeness of the Apples
This might seem weird, but take this into consideration: Not only would the very pony that kickstarted Ponyville have to move, but also a holder of an Element of Harmony. I don't think Celestia would approve of splitting up the Elements.

The Celestia toy is Celestia's evil form
Hence its a garish pink, and says vapid and vain things.

Princess Celestia actually isn't the Chessmaster the fandom likes to make her out to be.
I'm willing to give her having had a hoof in arranging things for the return of Nightmare Moon, seeing how she had advance warning there (and may in fact have known how long the banishment would last right from the start) and would have had good personal reasons to want to be prepared. Most everything since, though, can just as easily be explained by her centuries if not millennia of job experience in dealing with surprises as they happen (while maintaining the Royal Canterlot Poker Face for the benefit of her subjects, of course).

Celestia did not banish Nightmare Moon the first time.
Godly power aside, nopony can wield The Power of Friendship alone. Nightmare Moon was actually banished by the Mane Six during a time-traveling escapade. Celestia took the credit in order to prevent a time paradox, and used this knowledge to seek out the Mane Six and arrange for them to get together, hence how she knew that Twilight would easily find five other Element Bearers in Ponyville.

In "Royal Canterlot Wedding", Princess Celestia knew that Cadance
She and Luna knew about the, and knew that they would be attracted to the Royal Wedding, so that was why security was so stringent. She noticed her niece acting strangely and watched her, but it wasn't until Twilight arrived that Celestia knew for certain. The only reason she didn't do anything about it at first was because she wanted Twilight and Shining Armor to learn to trust each other more.

She didn't tell Luna about Cadance because she thought that the socially awkward and formal pony would overreact and spoil the plan.

Princess Celestia is extremely out of practice when it comes to combat.
Which is why she lost so badly in her only on screen fight. A thousand years of benevolent rule really doesn't help one get ready to rumble.

Celestia was holding back in her Beam-O-War with Chrysalis
Remember the world made of cardboard? Celestia is that powerful. If her attack was too strong, it would have destroyed Canterlot Castle and killed everypony inside. She believes that her own defeat with little destruction will be preferable to defeating Chrysalis with massive destruction. This will probably lead to an I Am Not Left-Handed moment where she does unleash her full power out of sheer desperation.
 * Alternately, the Beam was a telekinetic desperation attack. Celestia's sun-based powers are more likely the Death From Above / Kill Sat type, using those indoors is out. She probably could just teleport Chrysalis to the Sun, but with Shining Armour in play she would need her full attention for that. She was trying to overpower Chrysalis for long enough to cast something nasty and underestimated the boost from The Power of Love.

Princess Celestia is the Invisible Pink Unicorn
The toymakers keep screwing up and making the pink visible.

Luna turned evil for the reason not written accurately in legends?
In another way to say, legend covers some facts. I won't say that the legend is fake, but possibly the legend covers the key element and just gives official explanation. Yes, official explanation: It means that it is not the true story behind.
 * Remember Celestia's nickname: "Trollestia"? Well, Luna is maybe kind of too serious, but Celestia keeps trolling her and even goes all "u mad?". She gets pissed off and thus rebels. In the beginning, things are not yet out of control so Celestia doesn't really care. But, things go out of control and it doesn't even work after Celestia apologizes (insincerely), so in the end Celestia has to seal her. But well, seems like Celestia didn't change much after 1000 years, while Luna has found out that she is the one who is too serious. Actually to be fair, Celestia does do a little of reflection and remorse, so she does change a little.
 * or maybe Luna was instead the troll, while Celestia is the one who was way too serious. Well, getting pissed off, Celestia banished Luna. She found out that she is really too serious and cold, and without Luna, it is really boring. That's how "Trollestia" is made.
 * How to explain Nightmare Moon? I guess it is possible that Luna doesn't think that her joke is overboard: instead, she is pissed by the thought that most ponies are humorless. You can say Luna goes over-cupcake, worse than Pinkie Pie's(in Party of One). But, for some reason, after she is snapped out of the nightmare(Nightmare Moon), she finds out that she is terribly wrong.
 * This part is possibly Jossed in one or another way.

Luna is currently working to try and improve her image.
Fans have speculated that Luna is busy taking her duties of controlling the night again. Now, there's no reason for her not too, but assuming that works the same as Celestia's magic, she should be able to go with her during princess duties. (Like say, she should have been there during "Swarm of the Century" and "Sonic Rainboom".) This is because Luna understands that she's spent a thousand years as one of Equestria's boogeyponies, and even a pardon from Celestia won't change that opinion overnight. (And even if it did, Luna would probably want to redeem herself anyway, assuming that she's The Atoner.) Luna is currently traveling Equestria, doing as many good deeds as she can. She may or may not be using her Nightmare Moon form for this, mostly because Nightmare Moon would make a good Batpony.

Nightmare Moon and Princess Luna are not the same pony.
'Nightmare Moon' is actually a sort of magical symbiote that manipulated Princess Luna's jealousy and resentment to control her. She was the blue mist that was impeding the groups journey through the Everfree Forest, and she feeds off of negative emotion, which is why she wanted it to be eternally night - so she could feed off of Equestria's fear. She's not as powerful on her own, though, which is why she had to take control of other things, or create illusions, instead of just outright stopping the ponies. As for why she didn't use princess Luna's magic, She could still control Luna when they were first released and the resentment for locking her away was still strong, but after 1,000 years alone Princess Luna missed her sister too much for Nightmare Moon to control her, until the ponies got close and Luna was afraid of being trapped again.
 * Aw, man, you stole my idea! I'm writing a fic about this. I propose that the Nightmare entity could only magnify Luna's natural bitterness and resentment, and isn't actually intelligent; Luna was the one turning into blue mist, but Nightmare was the one causing her to berserk. I believe the black armor laying around Luna after the rainbow attack was the Nightmare entity's physical form -- it was evaporating into blue mist. It may not be dead...
 * Expanding on this, who have we met who has natural power with magic, is feeling jealous and resentful towards a member of the main cast, would leap at the chance for more power, and is something of an Ensemble Darkpony? I'm guessing sometime in the future we'll see the rise of The Great and Powerful Nightmare Trixie.
 * Assuming that the Nightmare entity is sort of like the Venom Symbiote, then wouldn't it gravitate towards Twilight as a host? Raw magical talent, with two dozen spells at her disposal, and she's being buttered up by Celestia for some reason. Draw a couple parallels, pull a couple threads, and I'm sure you can find a story...
 * Well, at the time, Celestia was closing in. It was defeated and weakened by the Elements of Harmony. Had it attempted to take over Twilight, it likely would have been destroyed by Celestia when she came. Alternatively, it could only be able to effect certain ponies like Luna due to her bitterness.
 * This idea makes way too much sense. It helps explain the Easily Forgiven aspect of Celestia's reaction. It also fits right in with the Magical Girl genre that the pilot seemed to be evoking.
 * Apparently confirmed by Word of God
 * And Luna spent a thousand years on the moon trapped inside that... creature, for what it did while controlling her. Nightmare Fuel indeed.
 * I can. Because Luna can turn back into the image of Nightmare Moon now (even though she's still herself) is a constant reminder of what she was subjected to. And even though she did it to spook everyone in jest, if this is true, she probably spent the night at Twilight's crying her eyes out.
 * I'm going to take this guess one step further: The evil force that turned Princess Luna into Nightmare Moon is none other than... THE SMOOZE.
 * Even better, it's Tirac. "Behold the power of darkness!", anyone? You can't tell me some kind of Whole-Plot Reference to "Rescue at Midnight Castle" updated for the 21st century wouldn't make for an epic season finale.
 * It has to be said. The entity that possessed Luna was of course Nyarlathotep

Luna won't be at the Grand Galloping Gala because she'll be busy working.
The Gala is on the 21st of an unmentioned month. The 21st of December is the longest night of the year. The episode that takes place during the Gala is titled "The Best Night Ever" (emphasis mine). Coincidence? Perhaps not.
 * Well if Luna's job involves more than just raising the moon, than she may have been busy that (and every other) night. But judging by the lack of snow, I don't think it was December 21st.

Spark-per-spark, Luna is more powerful a magic user than Celestia.
When Luna, as Night Mare Moon, enacted her rebellion, her magic was strong enough to block Celestia from rising the sun. Luna herself was so powerful, Celestia had to resort to an external source, the elements of harmony, just to be able to contain her. This, of course, assuming Night Mare Moon was a fabrication of her own magic, and not an external source as well.
 * Possibly correct. Magic would seem to fit the lunar portfolio better than the solar one. On the other hoof, maybe it's just easier to hold everything still than to force the Sun to rise.
 * Even if it was easier to hold the celestial position, it still took effort to do so under contest from another force. So since Celestia at her 100% full undivided power was not enough on its own to even subdue a distracted Luna, the point still stands.
 * Alternately, Luna was able to block Celestia's magic not because of raw power, but because of metaphysical Rock-Paper-Scissors: the moon has the ability to block the sun, while the sun can only make the moon shine brighter.
 * Alternately, it could be a simply case of Weak but Skilled. While Celestia is stronger than Luna, she had not only far more control with her power and knew thousand of different battle spells (like a proto battle wizard), but thanks to the inherent "propieties" of the Moon with the Planet, she could alter far more direct and profound Equestria that Celestia ever could.

Luna occupied her thousand year sentence on the moon by building.
Between her powerful magic, unlimited building materials, and centuries of free time, Luna eventually took up building while on the moon. At first simple structures for practice, perhaps a small village of houses built out of moon dust. With generations of experience, she began larger projects. skyscrapers, statues. Eventually, setting her sights on something to call her Magnum Opus, a gigantic temple built out of large, intricately carved moonstones. Sprawling for miles in all directions with fully glassed windows made from fired moon dust and shaped like a giant unicorn head visible from earth, it became her home for the rest of her sentence. Unfortunately, gravity back on terra firma made all her building experience worthless when she returned, as everything she learned to build would collapse under its own weight in Equestria.
 * So basically, she spent a thousand years playing Minecraft.
 * Some people could happily play Minecraft for 1,000 years. It's that addictive.

We'll get an episode where the main cast joins Luna on a trip to the moon.
It'll involve a lot of soul searching for Luna and the main ponies forgiving her. Also, we'll probably get to meet Luna's moon rock buddies. The reason for the trip? I dunno.
 * You need a reason?
 * In the same episode, Pinkie pie will befriend a hyper, sentient satellite. Who may actually be space core.

Princess Luna will become a villain again.
Because nothing has changed. In a climactic Motive Rant, the lack of her being mentioned at all through Season 1 will become a plot point, showing how the ponies still don't appreciate her even when appropriate (such as during the meteor shower), as well as the number of events that Celestia attended but she wasn't able to (since she has to sleep during the day). Adding onto that will be an accusation leveled at Celestia, a completely accurate one: that she only ever talked about things and never actually did anything to resolve their problems. This will lead into the lesson that attempts at reasoning and offers of friendship are not always enough - you have to actively solve problems rather than trying to convince people to ignore them.

During the Gala, Luna was hiding as part of a plot by Celestia to liven up the Gala in case Twilight and co. couldn't.
Celestia managed to convince Luna of the wonders of trolling, and the two hatched a plan: if Twilight and her friends were unable to liven up the party sufficiently, then Luna would stage a return of Nightmare Moon. Then one thing would lead to another and they'd have a great laugh about it afterwards.

Princess Luna has been spending the first season FIGHTING CRIME!
Batmare!
 * If only. If only...
 * Da Da Da Da Da Da Da Da BATMARE!

The reason Princess Luna wasn't at the Grand Galloping Gala was...
She simply didn't want to go. Let's consider that Celestia remarked that the Gala is "Always terrible", and it seems to be a stuffy posh event of the obnoxious aristocracy. It's likely that...
 * These ponies might have been very cruel to poor Princess Luna
 * Unlike Celestia, Luna had no reason to be there. Celestia was expected, to greet everypony who entered. Luna was under no such obligation.
 * To make up for her absence, Pinkie will throw Luna a much better party, Pinkie Pie style
 * A Pinkie Pie party for Luna? Sounds reasonable to me. Especially if Luna ends up spending a lot of time with the mane cast in season 2.
 * Celestria prank: during that night Luna was busy to make all her Moon God job quick and go to bed so she can rest befor Gala next day, but after she wake up she find note from her sister (sended when Luna was asleep) telling that Celestria move Gala on this night just Luna, so Luna learn about this after Gala.
 * This troper assumed that she was still resting. After being possessed (and on (in?) the moon) for a thousand years, you might want to spend a few months refraining from excitement. Maybe.

Princess Luna is still evil.
Her supposed "redemption" is actually the first step in an elaborate Evil Plan to seize the throne of Equestria.

Princess Luna is training offscreen in how to be a Princess
The reason why Luna hasn't appeared so far is becasue she is trying to train herself in how to show a non-godly visage to the population. After recovering from the Elemental attack, she and Celestia realized that Luna has no experience how to act with mortal ponies, at least in a way that doesn't result in instant idolatry/mass hysteria. While both of them are immortal, slepless, hungerless (how otherwise she could survive in the moon), multi time existential, near omnipotent, omniscient (day/night) beings who mere precense could crush a mere pony in seconds; after Nightmare Moon Celestia worked hard for eons in how to repress and appear only as regal and noble instead of Godlike, wile Luna at her best can come as quite... "accidentally disturbing", like creating matter out of thin air (food, houses, palaces and geographic locations), reading minds and dreams, remembering things from the Future, mass population teleportation, existing in two planes of reality simultaneously and so on. After a year she is sent to Ponyville to show her results to the mane cast and is what kicks the second season plot.

The Great and Powerful Trixie was in fact Princess Luna in disguise.
This is why we didn't see Luna for the rest of the first season. She had been away a long time, needed to get an idea of what the new social norms were, and she's been doing it by observing ponies without their knowledge (even taking on a jerk persona in order to remain hidden).

Trixie was only one of several disguises she wore. When you see an odd clone or a recolor in the background, it's her.
 * Photo Finish, Sapphire Shores, and most of the other one-show characters who actually moved the plot along were real, however.
 * So Princess Luna pretended to be incompetent in handling an Ursa Minor, leaving Ponyville in danger, just to allow Twilight Sparkle a chance to shine?

Princess Luna has spent the entire first season in hiding
For someone who really cares about people, being forgiven is one thing, but forgiving yourself is much harder. Between guilt at having become Nightmare Moon and fear of being shunned by pony society, she's spent the time since her return hiding out in her room. Celestia has been sharing Twilight's letters with her in an attempt to coax her younger sister out of her shell.

Princess Luna was at the Grand Galloping Gala.
She was there, but just off camera. She wasn't at the doughnut shop later because she had remained at the Gala to see to the other guests after the disaster. She may have been present for other events during the season as well, but just off camera.

The dangers the ponies faced in the pilot were Luna Fighting From the Inside, not Nightmare Moon's original plan.
Nightmare Moon was more than powerful enough to defeat a little gang of ponies searching for the resting place of the elements, but instead she elected to send obstacles in their paths that ironically helped Twilight put everything together at the end to activate the Elements. Rather than these simply being Nightmare Moon attempting to scare them off and failing, each of the obstacles was actually a Secret Test of Character Luna was able to put together in hopes that a pony could overcome all of them and therefore be able to use the elements when they reached them. The Nightmare Moon persona was unaware that she was actually enabling a Nice Job Fixing It, Villain moment, but Luna had set the entire second episode up in hopes of freeing herself from whatever it was that had transformed her.
 * Likewise, Celestia only defeated Nightmare Moon 1000 years ago because Luna was Fighting From the Inside. Luna was pretty much able to overpower Celestia with ease twice. Centuries ago, Celestia was Luna's size, apparently much younger and less experienced. Celestia was on the verge of defeat when Luna held Nightmare Moon back and let Celestia seal her away, she may have even told Celestia to stop her. Celestia imprisoned her instead of destroyed her because she couldn't bring himself to kill her sister.
 * For a slightly different take on this assume that Celestia and Luna can do anything any of their subjects can do, except better, and that the corrupting influence could only twist Luna so far and had to pick its battles. The first means that they are precognitive (since Pinkie Pie is) to a degree, and the second means that Luna was planning to keep all the ponies alive with magic when the food ran out (a task even her massive magical abilities would probably be insufficient to). Thus Luna/Nightmare Moon wanted to WIN, but she didn't per se want to kill anyone. Humbling an enemy before you in a submission they won't renege on is a more perfect victory than simply killing them. Luna had JUST enough doubt about the plan that the question she was asking her precognitive abilities when choosing between a wide variety of potential approaches was always "When this is all over, will I feel happiest on this specific course?", rather than "Will I win?". She used superior judgement to eliminate the possibility of a hollow victory, but since her precognition was seeking out a very specific fact, rather than showing a broad picture, it gave misleading results. She probably recognized that she was creating a Plot Tailored to the Party but assumed that it was going to end with Twilight FAILING to summon the sixth element. Even when they charged eachother, Nightmare Moon was planning on going for a leg to cripple, while Twilight's own horn would have been either parried, or bounced off her magically defenses or goddess-tough skin harmlessly. She was so sure she had achieved check-mate when she smashed the element-stones (possibly despite knowing it didn't actually stop the elements from being activated, since she knew that TWILIGHT didn't know that) that the arrival of the other five ponies and the activation of all six elements, combined with the "illogical" information ("You will be happier if you don't teleport away.", "You will be happier if you don't spear the purple one through the jugular.", etc) she was getting from her precognition that she suffered a brainlock that resulted in her just standing around while it happened. Then Celestia returns and offers her back everything she had lost when she was banished to the moon. All as Celestia's own precognition had predicted. So basically, it wasn't JUST a Crowning Moment of Heartwarming... it was a Crowning Moment of Heartwarming AND the sort of brilliant strategy you would expect from a ruler with many centuries, if not several millennia, of experience.

If need be, Luna can summon Ursa Majors to do her bidding.
Because it would be cool.

Luna allowed herself to be locked up in the moon.
In a moment of weakness, when she was off guard, she was taken over by the evil force against her will. The real Luna was still inside, almost completely helpless to the force. But Luna knew the force was more powerful than Celestia, so she resisted the force long enough for Celestia to take action. Even with Luna resisting, Celestia couldn't purge the force from her sister, so she had no choice but to imprison her in the moon until she could find a way to defeat the force. Celestia was heartbroken and spent centuries trying to find just the right ponies to wield the Elements of Harmony. She knew that six different ponies wielding the elements would be stronger and more powerful than just herself. (since the ponies have to be in HARMONY with each other for them to work) She managed to find the right ponies, all linked together from an early age by the Sonic Rainboom event to begin with, just in time before Nightmare Moon escaped. They were able to purge the evil force successfully and Celestia and Luna were reunited.

Discord is responsible for Luna becoming Nightmare Moon.
Think about it - first, Luna and Celestia take notice of how Discord is making Equestria a terrible place to live. They find the Elements of Harmony to stop him. Discord recognizes that he's unable to overcome the elements when they're being used against him.

So before the two turn him to stone, he takes advantage of Luna's place as being in charge of the moon and being the younger sister, setting Luna up for her Face Hoof Turn, knowing that it'll cause Luna to go bad and that Celestia will be forced to banish Luna. This in turn sets up Celestia to loose control of the Elements when she's forced to rely on other ponies to become connected to them, allowing Discord his opening to escape and a second shot at victory.

What's more, as long as they're given to ponies who aren't as immortal as Celestia and Luna, it means from now till the end of time he'll have a new opportunity once per generation to get free when a new set of ponies is needed to wield the elements and the spell is broken again.


 * I KNEW I couldn't have been the only one thinking this! After seeing Discord break and Mind Rape five of the Mane Six and remembering reading that Lauren Faust said that Luna had been turned into Nightmare Moon by evil magic, it wasn't hard at all to imagine Discord played with Luna's insecurities of being in charge of the time phase when everypony's asleep shortly before his first defeat. And while he was defeated anyway, the seeds of doubt were already sowed...
 * Though, when Discord was defeated in the modern days a World-Healing Wave undid all the effects of his magic. Assuming the same happened last time (and it MUST have, otherwise Equestria would still be plunged in leftover chaos), how come it did not undo whatever he did to Luna? And if there was actually no magic involved, then how do you explain Nightmare Moon being distinct from Luna?
 * It is possible that Celestia's relationship with Luna being weakned allowed Discord to cast a single spell while sealed, as the elements of Harmony were connected with Luna and Celestia back then, and Discord did claim to have been awake while sealed.
 * Strange minds think alike! As soon as I saw Discord, I totally thought that too!

The reason Luna hasn't appeared is...
She's simply nocturnal. Very few of the episodes have taken place at night, even fewer (two) in Canterlot. Luna is the one in charge of the night, Celestia the day. The reason we see Celestia all the time is the majority of episodes take place during the day, when Luna is asleep. When Celestia goes to sleep, Luna takes over as acting royality through the night and governs Equestria during that time. The two sisters spend time together in early mornings and late evenings (twilight) when both would be awake. As for why she wasn't at the Gala? She's younger physically than her sister, whose to say she's not younger emotionally and simply wouldn't like going? As for how Celestia managed to do this alone for 1000 years, she probably just slept and was awakened if anything came up. Luna, being the younger sister, would've naturally been tasked with less things than her older sibling anyway, so less offical stuff probably happens at night.

There will be an episode detailing the events that led up to Luna becoming Nightmare Moon and being sealed in the moon.
There's a TON of WMG on the issue already, it's a big topic of discussion among fans. Not only could it be a cool episode, it would please a lot of fans who really want to know the details and clear up all the guessing on what happened that night, AND give more insight on the relationship between Luna and Celestia.

Luna is a Nightmare Fetishist.
In the upcoming Luna episode, Luna will be trying to change the scary image she has in the eyes of the other ponies in Ponyville. This will be difficult for her though, not just because of her time as Nightmare Moon, but also because she actually really loves the dark and the macabre. Not because she's evil, but simply because it intrigues her. As a goddess of the night, she's naturally drawn to dark, spooky things.
 * Close, but it's more that she comes off scary as hell rather being interested in scary things though you could see it both ways.

Luna will be presiding over Nightmare Night because it's a Nighttime event
Now that Luna's back, she wants to resume her previous role of bringing forth the night, and by extension presiding over nighttime events. Furthermore, whatever ceremony or event she presides over will be spectacular, greater than anything Celestia ever did when she presided over these events in Luna's place. That will help gain more Ponyville goodwill.

Luna is every bit as mischievious as her sis, but prefers scares over pranks
Celestia has a mischievious streak, whose to say it's not a family trait? Considering Luna's Big Entrance is decidely frightening and one of the clips shows what's apparently her taking on a scary form to frighten the mane cast, it seems she likes giving people a good scare, but mostly just as harmless fun. On this note, it'll be explained that she invented Nightmare Night in the first place due to this belief.

Luna is a Goa'uld
Come on, you heard that voice.
 * Don't ya mean Tok'ra? (And if you're thinking of trying to spin it as nightmare fuel-why would a Goa'uld who's already established herself as a goddess act as if mortals actually exist?)

The reason Luna's mane changed between seasons...
... is that it reflects her general power level at the time. Bear with me here, this is different than her regaining power between seasons. We've seen lots of fanart with a younger Celestia having a plain pink mane. Perhaps her mane has its celestial glowy-ness during the day, when it would make sense for the, y'know, sun goddess to be more powerful. In turn, Luna's mane would be plain during the day and all glowy at night, since she'd have more power at night.
 * Can't be. Celestia's mane is still rippley like always at the end of "Lesson Zero", and it was nighttime, not to mention doing the greeting at the Grand Galloping Gala. Luna seems to have simply recovered completely or almost completely from her time as Nightmare Moon, and regained her natural powers, which went unused and weakened as Nightmare Moon.
 * My own guess is that Luna's first appearance look and Celestia's pink mane look are actually they're Sleep Mode Size. Taking this shape may actually also be their substitute for sleeping. Celestia's always in her fully powered size (at least when seen in public) because no one would recognize her otherwise and Luna stayed in her Sleep Mode Size at the end of the pilot to avoid scaring anyone but went full powered in Luna Eclipsed for the same reason she was using the Canterlot Royal Speech, because she wanted to address others.

Luna is a female alicorn version of Thor.
She's loud, speaks in Ye Olde Butcherede Englishe and has a tendency to cause random lightning and thunder.

Star Swirl the Bearded was Luna's apprentice
This is how she knew about him, who ever he was.
 * Could just be that she knew of him when he was making it big.
 * Seems Jossed by "Hearth's Warming Eve." Twilight Sparkle, as Clover the Clever, said Starswirl was her mentor, and the play is specifically said to pre-date Celestia (and presumably Luna's) rule over Equestria.
 * That they weren't actively ruling it (and they couldn't very well have been if Equestria hadn't even been founded yet, anyway) doesn't necessarily imply that they weren't already around in some form. In the absence of clear canon information about the origin and early history of the eventual royal sisters, we just don't know one way or the other. (Fanfic writers, of course, are having a field day with that.)

All is not well for the Mare in the Moon
When asked by Twilight if it was a good thing that she was purified by Harmony, she gives a kinda non-nonchalant "Yeah, of course. I've never been happier." Maybe I'm reading into this to much, but it seems like there still is something still lurking under the surface.
 * I think that was the point-she's rather thin-skinned throughout the episode, which could easily be read as her believing she can never really have the respect she wants. Which, given how she turned evil because nobody gave her the time of "day", really makes a lot of psychological sense.
 * Point, but I'm thinking more along the lines that she didn't entirely bury the hatchet with Celestia over what caused the banishment in the first place, and/or her raw id kinda liked being Nightmare Moon in a twisted way, if only for the attention and veto power over her sister. So there's nothing really stopping her from regressing back into the Nightmare shortly over the first big bump in the road, or at least picking up tendencies if things aren't going her way.

Luna is nocturnal.
Luna sleeps during the day, and Celestia sleeps at night.

Night Mare Moon was just a fictional persona that Luna made up.
She was just a scary, boogeyman-type villain she created for the Nightmare Night celebration, and she ended up Becoming the Mask when her jealousy (and that outside source of evil) took over.

If Princess Celestia is a horse incarnation of Amaterasu, then Princess Luna is a horse incarnation of Bayonetta.
You know it's true.
 * Depends, is Bayonetta a goddess in any way? (I don't play games like that so I wouldn't know).

Luna is the Equestrian goddess of death.
That is part of why she is so feared. She thinks of death as a rest and release, and as a necessary part of the natural order, so she doesn't understand why all the other ponies fear it, and hence her, so much.
 * Soo... she's essentially Equestria's version of DEATH?
 * No, he's an Anthropomorphic Personification, she's just a goddess. Equestria got along just fine without her whereas DEATH's Ten-Minute Retirement threw the entire Disc into chaos.
 * Original poster here. I would have said yes. The reason Equestria didn't lapse into chaos without Luna is the same reason the moon continued to rise at night: Celestia took over those duties.

Princess Luna created Nightmare Night herself
Luna's sense of humor is more towards the scary than her sister, so she invented a night time holiday centering around fun scares. On a related note, we'll find out Celestia invented the April Fools Day analog.

Luna wasn't banished to the Moon--she was banished to the Warp.
Nightmare Moon was a manifestation of the Ruinous Powers in Luna--it happens that the Warp Portal manifested on the Moon as a coincidence. After her freedom from the Ruinous Powers, Luna spent the rest of Season 1 on a crusade of penance, protecting Equestria from the enemies of Pony, seeking to atone in blood--that of the enemy, or her own.

The Reason for Luna's Mane-over.
The first time we see her (in episode 2) is during the day. Her powers may have been diminished. We will see her in Season 2 during Nightmare Night, in all her nocturnal splendor - hence the wavy mane.
 * On top of that, she was probably still weak after being hit with the Elements of Harmony and being returned to her normal form for the first time in 1000 years.
 * Celestia's mane doesn't do this- it's wavy at night and during the day. I think Luna's powers had lain dormant when she was Nightmare Moon, and became weak. When she got back to normal and regained use of her powers, she reverted back to normal and her mane returned to its glittery, wavy awesomeness. It doesn't seem to have anything to do with the time of day.

Luna was almost cured before her thousand years of imprisonment were up. Then came the astronauts...
"UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU... John Madden John Madden John Madden John Madden John Madden John Madden... ? ! ? ! ? ! ? ! ? ! ? ! ? ! ? !... Here comes another Chinese earthquake! Aeiou. Aeiou. Aeiou." It would be enough to drive anyone insane. See?

Luna played a role in defeating Nightmare Moon
. It's been proven that the Elements won't work if there isn't a strong bond between the wielders, so there's no way Celestia could've defeated Nightmare Moon singlehoofedly. Here's how I think it went:

Luna was told by the evil force that possessed her that everypony would love and adore her if she agreed to her terms. Luna agreed, but realized that this was a very bad idea very quickly. With Celestia helpless to do anything and Nightmare Moon repressing Luna's personality so she can't interfere, Nightmare Moon made the night last for years, causing many ponies to starve to death (as well as forcing poor Luna to watch and rubbing it in her face that it was essentially her fault). Eventually, Celestia could no longer stand to watch her people suffer, but also couldn't bring herself to betray her sister. When she finally decided to confront Nightmare Moon, the Elements failed after her first attempt. To Celestia and Nightmare Moon's surprise, however, Luna managed to take control of Nightmare Moon long enough to help Celestia banish Nightmare Moon to, well, the moon, knowing full well that she would be trapped with her. After a tearful goodbye between the sisters, Celestia and Luna used the Elements of Harmony to defeat Nightmare Moon, sending Luna away with her. As such, I believe Luna was The Atoner long before she was saved by the Mane Six.
 * I expect to see this fanfic on my desk by Tuesday.
 * OP here; it's funny you should mention that, because I was intending to put that in a fanfic. I just don't have the time to do it at the moment.

Nightmare Moon is capable of invading ponies' minds
Nightmare Moon had a much easier time fitting in with modern Equestria and she had a custom-made temptation for Rainbow Dash. Meanwhile Princess Luna, despite having over a year to learn modern customs still had trouble. Nightmare Moon invaded the minds of the modern Equestrians for modern speech patterns and had sufficient confidence to use them.

The monsters in the Everfree Forest are part of Luna's purview.
Luna's appearance on Nightmare Night indicates that she has a strong affinity with the scary and macabre. Being responsible for looking after all of the scary stuff in the Everfree Forest seems right up her alley. Also note how the Everfree Forest has gotten less and less dangerous ever since Luna came back, from "Nopony who goes in has ever come out" to "Going to visit Zecora, brb". Part of her royal duties have been to get the place better under control.

Luna's appearance changes between season 1 and season 2 because she has "phases," just like the moon
It makes sense, doesn't it? Her kind of small, weak-looking form in the second episode could be her "new moon" form (which actually fits very well with her "newly redeemed" status at that moment) while her taller, more powerful-looking form in "Luna Eclypsed" could be more like a half moon or even a waxing crescent.
 * Alternatively, as run with in the popular Fanfic Past Sins, an Alicorn's maturation is a Mana Meter and in Season 2 she's just regained some of her lost power, after over a rest period of who knows how long, growing from what looks to be a 13-year-old-like body to a 18-or-so one, comparing assuming her full power would be something as large as Celestia.

Twilight's findings about Friendship are being used for Luna's rehabilitation.
Initially, I took Celestia's giving Twilight Sparkle the mission to study friendship a thinly veiled way of giving the dutiful pony what she wanted but would never actually ask for. But on reflection, another idea occurs to me. Celestia's proven to be the type who never has just one angle. While Twilight's "mission" is partly a reward, it does in fact serve a practical purpose. Twilight Sparkle's Aesops are being used with Luna to help rehabilitate her and encourage her to reintegrate into pony society.
 * Building on this, it wasn't Celestia that was sending the letters to Twilight when she had been corrupted. It was Luna, who knew what it was like to fall victim to your darkest feelings.

Luna really REALLY hates Celestia.
It's pretty much fanon, and they've never been seen on screen together, just once Luna collected her mind after the whole Nightmare Moon debacle, she realized how HORRIBLE Celestia is.
 * Jossed by 'A Canterlot Wedding'. Celestia & Luna are not only standing right next to each other, they're getting along just fine.

Luna has a pet Moon Rabbit.
If Celestia can have a Phoenix for a pet, what's stopping Luna from having a mystical pet of her own?

Luna is fond of children.
Princess Luna has a soft spot for children. While her interactions with Pipsqueak (rushing to his rescue when he fell into the apple bobbing bucket and at the end) might be enough for this, the reasoning for this WMG is something else entirely. Luna's Face Heel Turn was motivated by jealousy and anger towards Ponies she felt were not appreciating her night. But, what's the one type of person who will consistently do all they can to stay up as late at night as possible? Children who want to stay up past her bedtime. Thus, Luna has a soft spot for kids because they make the most effort to see her nights than anypony else.

Luna is FAR more powerful than her sister
The sun is but the nearest star to a habitable planet, which Celestia controls. However, Luna controls the night, and all the MILLIONS OF BILLIONS OF STARS. Not to mention comets, asteroids and even other planets. Luna has the potential to rule the entire universe, but Celestia is keeping her in check. Not to mention, a direct hit from the Harmony rainbow of friendship would certainly mellow anypony out.

A cohesive theory for everything that happened to Luna.
The reason Nightmare Moon spoke normally was that Luna was dormant during her imprisonment on the moon and NM could observe the goings-on of Equestria and thus update her speech when she returned. Luna also spoke normally during her short line in the pilot because she was a) shell-shocked and momentarily forgetting the Royal Canterlot Voice, and b) still retaining some NM influence there, so it took time for her normal way of talking to come back. Later, Luna was going through rehabilitation, and still weak. She was bored during her lessons on the new Equestria, and when her flowy mane came back, she used some of her power to escape the castle one night, not knowing it was actually Nightmare Night.

Luna's appearance changes in time with the lunar cycles. Or the lunar cycles are dependant on Luna's appearance.
There's been plenty of different versions of Luna about, including her "Nightmare Moon" phases. Perhaps she cycles through them depending on the shape of the moon.
 * On the New Moon, she looks like a small filly, similar to Nyx from Past Sins.
 * When the moon is in the crescent phase, she looks like her form in the Pilot episode.
 * Half-moon phase Luna is the Luna from "Luna Eclipsed".
 * Full moon phase is her Nightmare Moon persona.

Of course, this can also mean that Equestria has been under a full moon for the past one thousand years, and is only just getting back into a lunar cycle.

Cerberus is Luna's pet.
Looks intimidating but is really friendly? Check. Has a somewhat scary but ultimately necessary duty? Check. And a spiky collar really does fit in with Luna's sense of style.

Luna actually sleeps in or on the moon.
"A Canterlot Wedding, Part 1" had her arrive from outside the force field around Canterlot to relieve Celestia for the night, and when she appears again at the end of Part 2 asking whether she's missed anything it seems clear that she must have spent the day somewhere else altogether again because she would have noticed the Changeling attack right quick if she'd been in or near Canterlot at the time. This would also help make sense of why Nightmare Moon was imprisoned on the moon instead of anywhere else: Celestia basically had nothing more to do than send her sister to her already-existing room and lock the door for a thousand years! (By extension, it's quite possible that Celestia's 'bed' is the actual sun, explaining where she was during most of the Season 1 pilot and why her return just so happened to coincide with the sun rising again.)

Mass guessing about where Luna spends her days, GO!

 * As mentioned above, the moon. After living there for a thousand years, it can take a while to move out.
 * Bonus Guess: Her Lunar Guards are ponies native to the moon, which is why we never see them without Luna.
 * As Celestia lives in a Minas Tirith expy, Luna lives in a Barad-dûr expy. After a thousand years of disuse and neglect, renovating it is a full time project for Luna.
 * She lives in her Luna Cave in a remote part of the mountain, too far away to hear the battle.
 * Canterlot. She was just stretching her wings in part one, and is a really, really heavy sleeper.
 * She was playing with Cerberus.
 * The Palace of the Pony Sisters. That's where she lived before her banishment, and she still feels comfortable there.

Luna actively participated in the battle for Canterlot.
She flew down from the castle and single-handedly beat down countless Changelings while defending her subjects. Why doesn't she remember it? Because she was sleepwalking at the time. Hence, she made her royal asswhooping with her eyes closed while shouting something incoherent about a petrol shortage.

== Princess Luna was added to "A Canterlot Wedding - Part 1" and "A Canterlot Wedding - Part 2" at the last minute. == There was so much fan outcry at her absence from the series in general, and especially her absence in the promos for the finale. The production team noticed these comments (How couldn't they?) and to avoid an Internet Backlash they quickly recorded a couple of lines of dialogue and threw her into a couple of scenes. In the first half she is only seen from a far distance and her line in the second half is a Lampshade Hanging of her absence. In both halves nopony even speaks to her and they barely glance at her.

Princess Luna was the primary fighter before her banishment.
Her general demeanor and temper suggest that she's quicker to resort to violence, and being known as a quick tempered bruiser would help explain why she more feared than her more graceful sister. Celestia's win-loss record also implies that she wasn't the stronger fighter.

Luna has her own palace somewhere else in Equestria, complete with her own capital around it.
Canterlot is Celestia's capital and palace, Luna's is elsewhere and is considered Equestria's capital at night. It's still under construction/reconstruction after her banishment and thus why she has to come to Canterlot to rule the Night at present.

Nightmare Moon isn't evil per se so much as a misguided yang to Princess Luna's yin.
Might there be a future episode where Luna might be forced to tap into that power even though she's apparently scared to death of it?

Winged unicorns aren't a naturally ocurring race - the princesses are just incredibly skilled unicorn sorceresses with a host of enchantments on top.
Note how they're called unicorns in the S1 opening. Also, unicorns used to raise the sun and the moon, according to the Hearth's Waming eve play. All it takes to become a princess pony are these spells: Their cutie marks are entirely natural. Eventually, unicorns were born that could control the heavens without needing the help of the other unicorns. The mightiest unicorn wizards (Star-Swirl if he was still alive) managed to cast a spell of eternal life on the sisters, so that the unicorns would never have to do it the old, hard way again. Their princess titles might have been given to them after that, or maybe they were born into the old Unicornian royal family. Their skills with other magic came much later; they didn't have anywhere near the talent with non-solar/lunar magic that, say, Twilight does, but have had several millenia to practice. By the time Luna went loco, Celestia had such a great understanding of magic that she could not only wield the elements of harmony single-hoofedly, she could also mimic her sister's cutie mark power.
 * Permanent wings - possibly similar to the spell Twilight cast in sonic Rainboom, except made to last. If a permanent version of that spell created the flutter ponies, who never seemed to have Rarity's fragility problems, Princess!Twilight might end up with butterfly wings.
 * Size-up - Maybe they had a chat with Tenser, or maybe some "ponies" just naturally grow that big (see Sapphire Shores and Fleur - might be a "giants walked the earth in those days" type dealie)
 * Longevity - might be a spell, or might be caused by the golden apple tree Celestia owns, like in Norse mythology.

Princess Celestia created Nightmare Night as a way to soften Princess Luna's image when she returned.
Princess Celestia always held out hope that her sister could be redeemed, and was always looking for ponies to wield the Elements of Harmony to save her once she was freed from the moon. In the meantime though, what better way to keep her sister's memory alive, while not completely retconning what she had done? Why not create a fun holiday around Nightmare Moon?

As a tribute to her sister's macabre tastes and love of the night, the holiday would involve dressing up in scary costumes and would be primarily celebrated at night. The legend of Nightmare Moon gobbling you up if you did not present her with candy could be used to promote Villain Decay and ensure her sister would have an easier time being accepted when she got back.

Even with the false starts, you will note that Nightmare Night was the perfect time for Luna to make her return. Indeed, it's quite possible that Celestia deliberately had Luna stay out of the public eye until then, conveniently explaining her absence in the show, because she knew that's when she would be best accepted. Just as planned!

Celestia and Luna became goddesses after defeating Discord (and related guesses)
I'm not sure if this is the correct page to put this guess on, and its actually a theory that is several different guesses that are related. Any way, here it is. As Discord is obviously an ancient and powerful god of chaos it only makes sense that there is a god/goddess of order called harmony. Earth's management of seasons and weather ect. seem somewhere between Celestia's rule and what happened when Discord broke free so it only makes sense that things are supposed to work on a balance between chaos and order. So in the beginning Discord and Harmony governed the world together, and there was balance, so things were much as they are on Earth. Eventually Discord defeated Harmony and attempted to destroy her, but as she was a Goddess she couldn't be destroyed completely and she lived on in the Elements of Harmony. This is alluded to when Nightmare Moon attempts to destroy the elements, but they live on in the mane cast. So anyway, now that Discord was in control things were even more chaotic than they where in the Return of Harmony, but still nature processes went on, just without any logic. Things really sucked and so the world needed to be saved. Discord heard somesort of prophesy that it would be an Alicorn who would defeat him, so he tried to kill all the Alicorns, but somehow Luna and Celestia survived the genocide. Together with a group of four other friends they found the elements of harmony and turned Discord to stone to avenge the alicorns. However, the destruction of chaos was not a good thing, as the natural chaos that kept things going ceased to exist and natural processes such as the sun and moon raising and setting stopped, so Equestria had a major problem. Celestria learned to manage the sun and Luna learned how to manage the moon, solving the problem, giving them their cutie marks, and turning the sisters into Goddesses. The ponies now had to manage the lack of chaos through order, which is why the ponies change the seasons and manage the weather. However, just as the defeat of Harmony didn't get rid of all order the defeat of Discord didn't get rid of all chaos. The area of Equestria that still had enough chaos to function on its own became the Everfree Forest, so named as it is the only region still free from order. Discord, being the cunning manipulator that he is managed to twist Luna to the point where he was able control her, and under his semi-possession which is also seen in the return of harmony he turned Luna into Nightmare Moon. Celestia couldn't purge Discord from Luna and turn Nightmare Moon back into her normal self as her party lacked Luna's element, and Celestia didn't want to kill Nightmare Moon as she was still Luna and still her sister, so she partly bound Luna using the five elements of harmony and banished her to the moon for a thousand years, after which she would be free, but a new set of bearers of the elements of harmony would be formed and they would be able to purge Luna. So the reason Nightmare broke free after a thousand years was not because there was some funky alignment of the stars that let Nightmare Moon break free, but because Celestia let her free so that she may be defeated and turned back in to Luna.

The Princesses are not actually sisters by blood
Rather, they're "sisters" in the sense of being female members of a formal group. Ages ago, there was a method by which normal ponies could magically become alicorns, and all alicorns were members of a mystical/religious/political group which ruled over Equestria and who took responsibility for the heavens. However, internal conflicts caused trouble and disaster, and eventually the group was whittled down until only two remained... and then, of course, one.
 * Supported by the Princesses looking nothing at all like each other except for body type (which, judging from the sign outside the beauty parlor, may just be the visual "shorthand" for pony beauty, similar to the "handsome" colt bodytype in the Gala episode). Most other instances of blood relation in the pony universe have shown ponies with similar colour palettes.
 * Not so much. Pinkie Pie is pink, while her parents were gray and orange. Twilight's color is darker then either of her parents'. There seems to be some random factor to how coloring is inherited. At the very least, the princesses could also have black and white parents.

The Canterlot Sculpture Garden doesn't actually contain any sculptures...
...They're actually ponies who were Taken for Granite by Celestia. Discord does appear to accuse Celestia of this with one of his lines directed at her: "It's quite lonely being encased in stone, but you wouldn't know that because I don't turn ponies into stone!"
 * They could all be, to a pony, completely deserving of such a fate. Capital Punishment is never considered.

Celestria and Luna are not responsible for raising the Sun and Moon....
....they are instead responsible for rotating the planet; assuming the sun is still 1.3 million times the size of the planet, it's a less impressive feat than actually moving the sun, but still godlike. This is the ONLY power ANY of the ponies have that are far reaching outside of Equesteria if the previous WMGs above are correct, with Celestria being forced to do double duty after Luna was imprisoned during her Face Heel Turn.

Celestia and Luna are refered to as Princesses because they actually are Princesses.
It's not just because girl focused cartoons generally have princesses instead of queens, it's because these two physical Gods, while ruling the kingdom, are still relatively lower ranking members of the local pantheon. The king and queen (or kings or queens) are either their parents or grandparents depending on how old they are and how high up they are on the Pantheon's food chain, are the creator gods, and, as this is a Hasbro property, may be this universe's manifestation of Primus and Unicron.
 * So the Creator Ponies are this universe's incarnation of the two eternally fighting brother entities. Wait... So Mom and Dad fight a lot before going to their room to "rest" for a few hours?
 * It seems that with the mythology theme pervading the series the creator gods are probably equine versions of Gaia and Oranos.
 * Then again, there could be a much more mundane reason for the titles; after all, a princess generally has to marry a prince in order to be considered a queen. Since Celestia and Luna both appear single, it makes sense that they'd retain their current titles (makes one wonder exactly what marrying one of them would entail, though, assuming it's even possible...).
 * Or it could be a buffer from Celestia to maintain her humbleness. While she is a God and quite likely the mightiest being in the Verse (probably second to old Luna [she did need the elements of Harmony to defeat her]), she realized that even a God can become drunk with power and adoration or the lack of it. So she declared herself a princess, a power to be instead of a Queen/Empress, who already has absolute power and created the entire Monarchy to divide ruling and responsibility and to mantain a tight check in her ever since.
 * Or it could be the obvious: Equestria is a principality, and who rules those if they're female?
 * Celestia and Luna are the princesses, Faust is the Empress

A twofer: Luna and Celestia aren't ponies, but horses; horses are basically "cave ponies."
Celestia and Luna achieved goddesshood before the rise of ponies, and bred/induced intelligent ponies from horse stock. Horses have either gone extinct, outbred and outclassed by the intelligent and magical ponies, or they are much rarer now and used as pets or servants. Actually suggesting that the Princesses are divine horses is likely to get accusations of blasphemy flung about unless you're in the right crowd. Celestia doesn't mind being called a horse (she is!), Luna is still a little self-conscious about it.
 * We see horses at the beginning of "The Best Night Ever", and they're apparently not even intelligent beings.
 * You mean the mice Twilight turned into horses? Those hardly count. They seemed to still have the brains of mice.

Celestia and Luna are the founders of all three pony races
These two are the only winged unicorns in the (apparent) world, and being at least 1,000 years old, they are also the oldest. One day, they decided to braid their manes together, The very second the manes touched, out tumbled the first pegasus, earth pony, and unicorn. That is how the first generation came into being, but once Luna was banished, Celestia had to instruct the ponies on how to do it "the other way".

Celestia and Luna are NOT all-powerful, immortal deities.
I realise quite a few fans would slay me for suggesting such a thing, but just hear me out. Think about how they make use of titles like "Princess"; regardless of their internal reasoning for doing so, such a title still implies a dynastic succession of monarchs, something which can really only happen if said monarchs are capable of dying (okay, I guess you could suggest that Celestia and Luna's parents simply stepped down or something, but where would they be now in that case? would we be expected to believe that the geriatric old King and Queen are rotting away in some isolated castle chamber?). I think it's entirely possible to assume that the Princesses, and by extension, all Equestrian monarchs, are something more akin to Tolkien's elves-- "enlightened" beings who wield extraordinary arcane powers and cannot die of old age, but are still vulnerable to disease, weapons, growing weary of life, etc...that said, the suggestion is not that they're any less powerful in terms of the things they can do-- merely that they are not one-of-a-kind, probably have parents who met some unfortunate end in the very distant past (remember, in the story told in the pilot, Celestia and Luna were already in charge a thousand years ago), and are only immortal in the sense that they cannot die of natural causes.
 * Possibly supported by the outcome of Celestia's fight against in the second part of "A Canterlot Wedding":.
 * They may not be all-powerful but they are clearly very powerful. We finally see an alicorn-level fight in the season 4 finale (Princess Twilight Sparkle wielding the combined power of all four royal alicorns vs. Tirek), and it looked like something straight out of Dragon Ball Z. They were literally punching each other through mountain ranges.

The Reason We Never See Celestia and Luna's Parents is because of Copyright Laws (spoilers for YU+ME: dream)
... for you see, Equestria is actually Nod. The current queen of Equestria was a very young girl who killed Don by mistake, yet was somehow still chosen by Nod to rule. She used her newfound powers to turn all the residents of the regular dream world into ponies (several thousand dream years has lead to them forgetting their true forms- after all, Equestrains don't seem to be very good at keeping track of history), found her prince charming, and had two daughters (who are the ones who actually have all the responsibility). But we never learn this, because in revealing her true appearance, we'd see her swirly eye and hair- and Rosalarian is not allowing them to do that for some reason or other. (why they don't just use a Lawyer-Friendly Cameo or the like is beyond me)

Celestia and Luna are the Nobody and Heartless, respectively, of Equestria's true ruler.
Equestria was, in fact, one of the first worlds visited by Xehanort when he did his experiments. He only succeeded in creating a single Heartless (and Nobody, by accident) before he was forced to flee.
 * Celestia, while still having incredible power, finds it difficult to find the motivation to do anything because she lacks emotion - which explains why the main cast has to do everything themselves. All Celestia really does is keep the balance of sun and moon, and also bar other Heartless from entering Equestria, because it's all she can motivate herself to do most days. She is, in fact, nearly emotionless but like most of the Organization, pretends to have true feeling because it's less unsettling and if she does it enough, she's convinced she'll believe it herself.
 * Luna, while retaining true sentience because of the original being's extremely strong heart, is driven by pure emotion and desire in the same way that Xehanort's Heartless in KH1 was. In this case, her desire is simply to win the Hearts of all of Equestria by having them celebrate the Night and Darkness which she now embodies. When that fails, she declares war and Celestia seals her away in the moon.

Celestia Didn't banish Luna to the moon, she only banished Nightmare moon.
Nightmare moon was the combination of Luna's powers and jealousy, since Celestia was unable to separate the jealousy by itself, she had no choice but to lock away the manifestation of her sister's powers. The Luna that was left behind was a feeble, barely alive husk, and was kept locked away in the royal palace to be cared for until her powers lost the evil taint. When Twilight and her friends wielded the elements of harmony, they successfully removed the Taint, and Celestia was able to re-unite Luna with her powers.

Celestia and Luna are the same pony.
"Celestia" and "Luna" are two sides of the pony goddess's personality. Celestia is the goddess's "light" side, which places a great emphasis on friendship, whereas Luna is the "dark" side, more self-centered, focused on the austere beauty of the night and looking inward to herself. The myth that tells of Celestia banishing Luna to the moon using the Elements of Harmony actually represents the goddess using the Elements of Harmony to strengthen her light side and repress her dark side, allowing her to usher Equestria into a thousand years of prosperity and friendship. This involves locking some of her essence away in the moon, in the form of Nightmare Moon. She allows the spell to last for a thousand years, hoping that in that time, she'll be able to find a way to reconcile her two sides.
 * In the modern day, Celestia knows that her suppression of her dark side is wearing off. "Nightmare Moon" is going to "break free", meaning that the goddess's dark side will be active again, and probably dominant after a thousand years of not having to fight against it. Knowing this, Celestia sets up the mane cast to recover the Elements of Harmony. When the spell ends and Luna becomes dominant, the goddess still retains enough of her light side to create the trials in the forest that establish the six ponies as embodying the Elements. When the ponies defeat Nightmare Moon, their use of both the physical (the rock thingies) and emotional Elements helps the goddess reconcile her two sides. The scene where Celestia and Luna embrace and tell each other how much they missed each other and how glad they are to be back together, is a visual representation of the goddess finding mental harmony.
 * Alternatively, they were once the same pony, but split apart into two, likely because of Discord. The two halves had to learn to cooperate with one another to harness the elements to defeat him. Also their original body is probably sitting in a cave somewhere, unthinking and unfeeling, just waiting for some villain to come try to harness its power...
 * Are you suggesting that Celestia and Luna are Reshiram and Zekrom?

Celestia and Luna are the only survivors of the "war of the heavens"
When Equestria was first made, there were bountiful alicorn gods and goddess that controlled all the natural cycles. One for the weather, the changing of seasons, the tides, caring for the animals, and so on. War broke out between these gods for some reason (Let's say the weather god and the seasons god got into an argument about when to start the coldness of winter) The gods all took sides, with the exception of Celestia, who only wanted to protect the then-infant Luna. The war killed everyone except Celestia and Luna, who were in hiding, and the severely wounded King of the gods, who had his powers significantly drained and now only controls what goes on in Everfree forest. Since then, they've had to teach the mortal ponies how to change the seasons, create weather, and all that other stuff.

Luna and Celestia have a brother.
And his job is moving the planet to ensure proper distribution of geo-thermal heat.
 * He's also Blueblood's Father.

Luna and Celestia are actually the same age
Its just that the sun is naturally bigger than the moon.

Princess Celestia proclaimed the Summer Sun Celebration out of regret.
Twilight tells Spike that the night of Nightmare Moon's return will be the thousandth year of the Summer Sun Celebration, which means, if Nightmare Moon's exile was exactly a thousand years, that the holiday was the anniversary of her banishment. Now, at the time, everypony probably thought it was a celebration of the return of the sun after Nightmare Moon's rebellion. But how do the ponies celebrate the holiday? By staying up all night. And why was Princess Luna so envious of her sister in the first place? Because everypony slept through the night and as such didn't appreciate her labors. So after banishing her sister, how did Princess Celestia commemorate the event? By commanding all her subjects to stay up all night long. In fact, I wouldn't be surprised if it wasn't a way of making her subjects atone for the sin of failing to appreciate Princess Luna properly.
 * This is actually a pretty logical idea. She knew the people probably wouldn't understand why Nightmare Moon did what she did, but knew they'd appreciate the sun's resturn after her defeat. So she disgused it while still atoning for her sister.

The Summer Sun Celebration is Celestia's alarm clock.
Instituting a celebration of the sun the same year that Celestia had to banish her beloved sister does seem rather tactless, doesn't it? There is, however, a good reason. Celestia knows when her spell will end (hence the "prophecy," which is really just notes on how the spell worked) and wants to have a reminder in place. So she started an annual celebration, putting on a show for everypony, and making sure that part of the tradition is keeping track of how many there have been. The whole festival came into being because Celestia couldn't find a countdown calendar that had a picture she wanted to look at for a thousand years.

The guards standing next to the throne in the opening sequence are Elite Guards.
Similar to Emperor Palpatine's red royal guard, these are specially trained guards, used as a last defense. Yet they never travel with her...
 * Because Celestia generally travels by air. It would be difficult to bring non-pegasus guards along and doing so would slow her down in general. The unicorn guards probably aren't any more elite than the pegasus guards actually, just each group of guards is trained to cover different areas and situations.

The Elements of Harmony are a system for creating new Alicorns by fusion of a Pegasus, a Unicorn, and an Earth Pony
Celestia and Luna were once the six previous holders of the Elements, and are cultivating, intentionally or not, the mane cast to follow them in Ascension, with Twilight Sparkle being given particular focus due to the keystone nature of Magic in the process. Presumably one of the new alicorns will be formed by Twilight, Rainbow Dash, and Applejack to create the Celestia-type "stronger and larger" of the pair, while the other will be Rarity, Fluttershy, and Pinkie Pie, forming the Luna-type "smaller and less raw power but specializing in a different vital function" of the pair.

Celestia trapped Luna in the moon when she was a young filly
Look at the intro in the first episode. If that's accurate, then look at their size difference. Celestia and Luna are the same size. Luna didn't get any smaller or younger, so Celestia just got larger and older.

The Mare on the Moon WAS just a legend
Celestia noticed that her beloved student had lots of potential, but was becoming more and more isolated from her fellow ponies.... by choice. No matter how much she insisted on her to get some friends, she basically ignored her and continued studying vigorously. She decided to pull a Batman Gambit and used the old legend of the Mare on the Moon, which actually spoke of her sister Luna, who ruled on the other hemisphere, just as her rules over Equestria. She pulled some strings to make Twilight meet exactly five ponies(That's why she gave the list) and gave her sister precise instructions to make Twilight "realize" that her new friends and herself carried the Elements of Harmony. While Luna acted her part, she was on the other hemisphere(Where exists a legend of the Mare in the Sun) moving the sun and moon. That's the reason Luna doesn't appear after this, as her visit was over and she returned to her country.
 * Alternant posibility. The Nightmare Moon legend WAS true, but the Mare On The Moon legend WASN'T. Celestia realized that Twilight and friends could free her sister (knowing they were meant to be true friends from the Sonic Rainboom before) and that the legend just so happened to coincide with the next day. So she asigned Twilight to read that particular legend, knowing her curiousity would cause her to investigate further and then freed Nightmare Moon herself so not only could Twilight have friends, but free her sister.

Both Princesses are Horses, Not Ponies
It would explain the longer legs and differently shaped muzzles

The Princesses' parents...
..were a white unicorn and black Pegasus. Their mother was a white royal unicorn with great magical powers, but was still mortal and her sister, their aunt, was the one destined to take the throne, and it is her bloodline where Prince Blueblood hails from. Since she was the second born princess she wasn't raised to be as regal and formal and spent her life travelling Equestria and the neighboring lands. She learned and possibly created many of the spells Twilight studies, including the false wings. She cast it on herself and went to a Pegasus town, possibly Cloudsdale and met a handsome black Pegasus stallion. They, hooked up, and the mixing of their magic blood created terribly powerful fillies, who than took over the royal house.

Celestia and Luna both are capable of emulating space-based weaponry.
Celestia can order some sunlight to redirect itself onto an enemy. Of course, she probably has to be in space to ask personally enough, and might need a ground-based spotter. One of her less known titles (in fact deliberately covered up for strategic and diplomatic reasons a while after the time she granted it to herself) is Hammer of Dawn. Luna herself is known as "The Lunar Lance" although nothing has ever come up where she found it necessary to drop a shooting star on anything in anger. Even when she was Nightmare Moon, she wanted to RULE, and thus collateral damage was undesirable. She was too demented to fully realize that no sunlight would condemn the world to death by starvation since crops couldn't grow.Both receive cooperation from their brother, Abyss, Lord of the Seaponies in that he clears out enough deep ocean of fish and such for them to do a little target practice with their respective weapons.
 * Also, if there was an alien invasion, Luna could just cause a star to go supernova and wipe out the enemy fleet before they even arrived. Celestia can do the same for the sun, but that would be an incredibly bad idea for obvious reasons.
 * Giving Luna the ability to cause supernovae at will in a regular SF-type universe would leave her capable of wiping not just fleets, but entire solar systems with all their inhabitants out of existence at a whim. Now imagine living in that setting and knowing that somewhere out there there's a temperamental pony goddess who just might decide to blow up your sun at any time for any reason. Sweet dreams...

Celestia and Luna aren't gods, just ponies with very rare talents.
Celestia and Luna hail from a time when alicorns were rare but not unheard of. They managed to learn how to control the sun and moon (or possibly even created them themselves), and that's how they earned their cutie marks. Since the ability to control the sun and moon is so rare, they had to find some means of immortality, otherwise Equestria would be without the sun and moon until another pony who could control them happened to come along. They use the golden apple tree mentioned in Best Night Ever to keep themselves young until they can find successors. They're currently desperately training Twilight and Trixie as their successors, so they can finally die.

Celestia and Luna assumed that Discord was dead.
They weren't experienced enough at the time to realize he might have survived. And by the time they were, they had good reason for assuming otherwise: Discord hadn't shown any activity for much longer than Nightmare Moon did, and when Nightmare Moon was banished and eventually came back, it was only one user of the elements, whereas with Discord it was two. In fact, you need six to take down Discord for good.

Celestia and Luna were Discord's secretaries.
I don't know, I just think it's funny.

Discord made Celestia and Luna
The original six bearers of the Elements of Harmony tried to stop him and he fused them into the two sisters for fun. Which backfired spectacularly.
 * Possible evidence: Luna referring to herself as "we" and implying that Celestia used to do the same. And yes I know it's just a royalty thing, in our world but not necessarily in Equestria

Celestia and Lunas respective elements
Celestia=Laughter Luna=Kindness
 * Since there were only two of them, shouldn't they each get three? Celestia= Laughter, Honesty and Magic Luna= Kindness, Honesty and Loyalty?
 * How about Celestia: Laughter, Kindness, and Magic. Luna: Honesty, Loyalty, Generosity. That way they each have an earth pony, a pegasus, and a unicorn element... assuming that the elements are each associate with a type of pony.
 * Ok that works but I ment the element they most embody
 * Its possible that there have been other Element bearers, but history only remembers the Royal Sisters.
 * One WMG somewhere listed the princesses as keepers of the EOH, joined by a Griffon, an Ursa Major, a dragon, and Discord.
 * I'm under the theory that Luna is Magic(Friendship), and there are four others for the other elements. This is mostly because Magic couldn't be found until Nightmare Moon was released from the Moon, and the fact that she turned evil because nobody would stay up and be her friend.

Celestia and Luna are mutants.

 * Their parents were a unicorn and a pegasus. Two of their grandparents were Earth ponies. While normally they'd be one or the other, some genetic mutation caused them to become Alicorns. Since all ponies are capable of using magic (with unicorns the only ones who can control it directly) this gave them extra magical power. At some point they found a magical fruit that drastically increased the power, and even made them immortal. They decided to use this newfound power to defeat Discord and take over as rulers of Equestria.

Celestia and Luna never wielded the full power of the Elements of Harmony.
That requires six ponies, one embodying each element. With just the two of them, they could only turn Discord to stone rather than get rid of him altogether, and carrying all of the Elements by herself, Celestia could only seal Nightmare Moon, not cure Luna. That was why she impressed the importance of friendship on Twilight Sparkle -- only a group of friends could bring her sister back.

Celestia and Luna have always been the supreme rulers and never "usurped" the throne. Discord's "reign" was merely that of unleashing havoc.
If anything, the land was in peace before he came along. Celestia and Luna (and maybe even their family, if they did have one) had always ruled as the Princesses over Equestria, and it was a peaceful reign until Discord invaded. Discord was never truly the former "absolute ruler" of Equestria to begin with. It was mentioned that he did rule, but that doesn't always have to mean being the actual ruler of a country. In this case, he was just responsible for a powerful reign of terror and anarchy among the ponies without any real sense of authority and there was no one that was able to stop him from doing what he wanted, because of his immense power and trickster nature. It doesn't mention that he actually HAD supreme control over all of Equestria, just that he had brought an "eternal state of unrest and unhappiness" with him. And before you mention Celestia and Luna stepping up to him, they had to discover the Elements Of Harmony first BEFORE they could petrify him. And since he became free, he is now plotting to take over as revenge for being imprisoned in stone.
 * Seriously, this was posted because the people over at the MLP:FIM Wiki seemed to jump to conclusions too quickly about Discord actually being Equestria's former ruler.

Celestia wasn't just protecting Equestria in the pilot; she was also protecting Luna.
If it had come to a fight without the Elements of Harmony Twilight stood a solid chance of killing her. The birth of a unicorn whose power rivals the princesses' is both a mystery and a closely guarded secret, so not even Twilight knows, but Nightmare Moon was able to tell; that's why she ran away and tried to stop the ponies indirectly.

After "Return Of Harmony", Celestia and Luna launched the Discord statue into space.
That way, if Discord becomes free again, he'll be far enough away that they can arrange for defenses before he manages to find his way back. Space is very big, after all. Luna could arrange for the statue to find an orbit that would make it easy for astronomers to keep track of it.
 * I wish that were true.

Celestia and Luna have empathic connections with the sun/moon and stars.
Let's say someone tries to Hurl It Into the Sun. Depending on what the object is, the effects on Celestia could vary from itchy eyes or a headache. Solar flares make her a bit grouchy.

There will be a Cutie Mark Crusaders episode featuring how Celestia and Luna got their Cutie Marks
Well we've found out how the Mane Cast got theirs, and "Call Of The Cutie" ended with Celestia reading Twilight's letter and the camera zoomed in on her Cutie Mark as she thought about it. It'd certainly be intresting.
 * Shoot, it would be interesting to hear more of their backstory in general. Hope they go for it.
 * There's still a possibility that the Mane Cast's stories from that episode were all or mostly fake, but yeah, that would definitely be a nice idea.

Celestia and Luna's parents were lost in a great battle
Their father wasn't cosmos, he was the Sky, protecting his daughters and their mother from the chaos of the cosmos. His power wore thin as time went on. Ultimately, he sacrificed himself to protect life on this planet. Earth fought back by supporting the sky, and lost a majority of her powers, save the Everfree forest. The eternal chaos was vanquished. Celestia Luna, and the ancestors of all ponies were forced to organize. Everything King Nimbus and Queen Tierra were responsible for, the seasons, the animals, the weather, The plants, were all failing on their own. Celestia and Luna were the only ones powerful enough to control anything beyond the sky, so they control night and day and some parts of the seasons. The Pegasus ancestor vowed to manage the weather. The Earth pony Ancestor vowed to maintain the plants and animals. The unicorn ancestor vowed to study magic, and learn to wield it in the most beneficial ways for their fellow equines.

Queen = Unified, Princess = Shared
The title of "Queen" implied that the realm is ruled over by a single individual. However, there is no limit to how many Princesses a kingdom can have. Celestia and Luna likely chose to restrict their royal title to such in order to divide Equestria's authority equally, thus preventing any animosity based on authority issues. Following Luna's banishment to the moon, Celestia retained her title of "Princess" for both convenience's sake and to symbolically retain the shared authority with her sister, awaiting her eventual return and reinstatement.
 * Your premise is false: a principality can have only one princess. While it is true that, in, say, Great Britain, all the Queen's children and grandchildren are referred to as princes and princesses as a courtesy, in fact only her eldest son Charles, as Prince of Wales, is actually entitled a prince. Prince Charles' elder son "Prince" William is actually a duke, specifically the Duke of Cambridge, not a prince. If she were Consul Celestia, then you would have a case.

Neither Celestia nor Luna are aware that Luna was corrupted by an outside force into turning into Nightmare Moon
Celestia and Luna both threat her return to normal as if she'd actually done something wrong, Celestia begging her to end their fighting and Luna asking for Forgiveness, and her upcoming appearence has her as The Atoner. Plus, the legend makes Luna out as a Green-Eyed Monster ultimately corrupted by her own bitterness. Celestia didn't write history wrong, she just wrote what she knew, that her sister got bitter and jealous then turned bad, not that she was corrupted by an outside force. Neither of them know that anything more than her being overwhelmed by jealousy was the cause. For Luna, it was simply a "what was I thinking?" reaction to her actions. She doesn't know why she acted like that but believes she did it of her own accord. They may have realized this afterwards when they could think it out or realize later when that outside force shows itself.

Twilight Sparkle is a descendant of the Royal sisters
Sometime before attaining Goddesshood, Celestia and Luna each bore foals. Celestia's foals were all white unicorns, and Luna's foals were all black or bluish unicorns. Several thousand years later, Twilight's Father is born from Luna's offspring, and her mother is born from Celestia's offspring. Never before have the sister's foals interacted.

Celestia and Luna have no parents.
They are the embodiment of cosmic forces, and so coalesced out of primeval chaos. They are sisters because they are the embodiments of paired and corresponding forces; also, the DNA of their physical bodies may be similar as would be normal siblings'. Corresponding to this, the reign of Discord was actually a period of primeval chaos and darkness, out of which the princesses created light and order, effectively bringing the universe into being. The story Celestia told to Twilight and her friends is just a highly simplified account of the creation, because only Twilight might be able to understand the more complicated version, and there just wasn't time.

Celestia and Luna are genetically engineered beings.
They were created by a group of scientists who wanted to make a superior being by using the DNA of many of the creatures that lived in Equestria's World. Luna and Celestia were not the only ones, but they were the most successful. Obviously they were made by combining the DNA of all three pony species and maybe even other creatures. Discord was one of these expiriments, made up of several creatures, making his nature so mixed up he could never think straight. Making everything else chaotic was the only way he could feel normal. He was extremely powerful, which is what the scientists wanted, but since he was so screwed up in the head he was deemed a failure. Like Mewtwo in the first Pokemon movie, he went crazy and broke out, and then proceeded to take over the land. After some time, when Celestia and Luna discovered the Elements of Harmony, they overthrew him, took over, and proceeded to fix things up and make life better for the other creatures in Equestria. The reason they didn't kill him was pity, they knew it wasn't his fault he had such a chaotic nature. They intended to find a spell that could help him, but their responsibilities to the world kept getting in the way. It got even harder when Luna became Nightmare Moon and Celestia had to do things on her own for a thousand years. One reason Celestia opened up the institute of magic was in hopes that another unicorn could find a way to help Discord. She couldn't continue as much magic study on her own anymore, but there are many talented unicorns and she believes they can do something. She questions if the magic of friendship can help, which is why she continues to ask Twilight for letters. Celestia, Luna, and Discord were far from the only beings created in the expiriments. There are other expiriments, with huge variety in appearance and abilities. Some may even be as powerful as Celestia, Luna, and Discord. They range from benevolent to malevolent, and we will see more of them in future episodes.

Celestia and Luna were the very first ponies to receive cutie marks
They did so when they ascended to godhood, and they then blessed the three races of unicorns, pegasi, and earth ponies with the ability to receive cutie marks.

THE TRADITIONAL ROYAL CANTERLOT VOICE was phased out after Celestia and Luna sealed Discord
With all the chaos and unrest during Discord's rule, Celestia and Luna needed a commanding voice to reach out to the ponies of Equestria, possibly continuing for some time after sealing away Discord. However, when everything was established and peace began to settle in, Celestia slowly began to tone down her voice and try to become more acquainted with the all ponies. Luna, however, could not adjust, which became one of the factors that turned her into Nightmare Moon.

The Princesses have two "selves" One to appear more regal, and their true form
The Grown-up horses are an illusion created by the sisters to inspire awe and obedience. However, they change back into their younger, less impressive selves in private. It's easier to do this during the princesses respective "times". Luna's magic is stronger at night, and Celestia's is stronger during the day. Celestia actually deeply struggles to keep up her royal facade during the Grand Galloping Gala, so she just greets everyone as they come in so she can retire to her chambers once all the guests have arrived. We have yet to see Celestia's true form, but the fan artists seem to have a pretty good idea of what it looks like.

The Princesses are Pure Magic Beings
Nightmare Moon spent most of Episode 2 immaterial or shapeshifted. It just doesn't seem likely that if Luna were simply possessed it would have that much power over her body. And considering that both Celestia and Luna have shimmering, semi-translucent manes it looks more and more like they only maintain a physical form for their subject's convenience.

THE TRADITIONAL ROYAL CANTERLOT VOICE and other archaic forms of speech were only used by the high court to address their subjects
Very much like how many languages have polite, formal variations of speech, only taken to the hammiest extreme. It is not, however, used by ponies of the same rank or family - hence why Luna apologies to Celestia in plain modern English (which to her would have been colloquial slang). Using Ye Olde Butcherede Englishe with a normal speaking voice would still have been considered polite, but lacked the noble dignity of the VOICE. Nightmare Moon, meanwhile, was boisterous enough but didn't have the pseudo-Shakespearean quirks, which would have made her seem haughty yet uncouth in her original time.

Guesses about the bat-winged ponies who pulled Princess Luna's chariot.

 * They were just ordinary pegasi; they were just wearing bat-wing costumes for Nightmare Night.
 * They are a different breed of pony from the pegasi, having bat wings instead of feathered wings. Seems unlikely, since we've never seen or even heard of such a fourth breed heretofore.
 * Something about Luna's magic automatically gives members of her retinue a more nocturnal appearance, appropriate to the Goddess-Princess of the night.
 * They're wearing enchanted armor that has a spell on it that works like the one Twilight cast on Rarity in "Sonic Rainboom" but is much stronger. Seems unlikely, however, given that the spell was so difficult that Twilight could only cast it once; a much stronger version would presumably stress even Princess Luna. Why go to all that trouble, when she could just hire naturally winged pegasi?
 * Perhaps the spell to modify existing wings is much easier than the spell to make them from scratch.
 * And if it is that easy (And Healthy!Luna is quite plausibly even more powerful than Filly!Twilight) there is no particular reason she couldn't cast the spell manually on each shift of guards and dismiss it when their shift ends. Depending on the exact details of how magic works in the setting, there may even be a "Transfer Spell Effect" spell, do she wouldn't have to re-cast it as often, even if it IS a difficult spell.
 * They're actually magically generated constructs, same with Celestia's identical white pegasi and grey unicorns.
 * Do we have any reason to think that Celestia's guards are magical constructs?
 * First they all look the same, second they only appear when she is around, and third Luna being Celestia's sister, have lived for a very long time and also a fellow alicorn seems to point out that the possibility that Celestia also has the capability to shapeshift and divide like her sister can is highly likely.
 * See response below.
 * They're half-dragon ponies. Normally, half-breeds are ostracized in Canterlot (if not by Celestia herself than by all those royal jerks at the Gala), but Luna, befitting her role, takes such outcasts as her own and gives them a ceremonial position of high standing in her personal court.
 * Alternatively, they're vamponies.
 * They're from Pranceylvania- the local Transylvania Expy. Pegasi from the region just happen to look like that; bat wings are a naturally occuring pegasus variant but a recessive trait, so we haven't yet seen the phenotype in other regions. Luna recruits all her staff from the area; later we'll see a unicorn pony in her service with a pale coat, crimson eyes, and a black mane with widow's peak (opera cloak and thick accent optional), and an earth pony with a massive build, a coat with various green and grey patches, and a shovel-like brow (scars and neck bolts optional). Like Luna, they're all actually quite nice once you look past their sinister appearance.
 * They are Pegasi who were changed by Luna's powers to appear more appropriate as the guards of the Princess of the Night. But they wouldn't choose the job "royal guard of the Night Goddess" unless they were Goths or Metalheads, and either way they think they bat wings are pretty awesome. Just look at the smiles when they pull her chariot!
 * They're from the moon.

The royal guards are actually magical constructs created by the princesses.
Each of it's members all look the same. That's very unusual for a non-background pony. They also almost exclusively appear whenever one of the princesses are around/nearby or have recently left. Why isn't there a royal guard any place else? Especially in Ponyville where it is in close proximity to an extremely dangerous forest. That's also very strange. Also Princess Luna as Nightmare Moon has demonstrated that she has the capability to divide and shapeshift herself into several ponies. If that is the case then it is highly likely that Princess Celestia also possess this kind of power. This opens up a new perspective whenever you see a royal guards: They are actually magical puppets and tools of the princesses to manipulate things and events to suit their plans.
 * Alternatively, they are royal guards. They may all look the same because they are the same size and have the same color coats and manes. The former may be explicable by the fact that there may be size requirements for recruitment into the royal guards. Historically, many militaries, especially elite units, had standard size requirements; the Roman legions, for example, would not take any man who was too big or too small. As for their coats and manes all being the same color, that may be the result of dyeing their hair, which, considering ponies don't normally wear clothes, may be their equivalent of a military uniform. Also, just because they all look alike to us doesn't mean they all look alike to other ponies. As for why they are always around when the princesses are, and not otherwise, that's because they are royal guards: their job is to guard the princesses and their household. They are not the national army and they are not the police. And it's not even true that we never see them when the princesses aren't around: in the pilot, two pegasi members of the royal guards flew Twilight's carriage to Ponyville, and she thanked them, as if they were thinking, feeling ponies; they smiled at being thanked, as if they appreciated it; if they are constructs, Twilight doesn't seem to know it.
 * Yeah, this one is jossed. Twilight's brother is a member of the guard.

The Royal Guard are ordinary ponies enchanted to look like that
. A Bird in the Hoof seems to confirm that they're not just made out of magic, since Rainbow Dash asked if she could join up. Implying three things, if RD isn't wrong: 1: They had a life before signing up 2: They get paid, so they probably have free will, days off and individuality 3: They accept females into their ranks, though we haven't seen any yet So, how come they all look the same? Magic. Celestia and Luna magically transform them into perfect soldiers (maybe only while they're on duty, keyed to their armor?), so they can focus on accepting the most loyal, rather than the most physically fit. This also makes them identical, so no-one can be sure who they are (for reasons similar to why SWAT teams and special forces wear masks), and might even change their race (this is why we haven't seen any earth pony guards, and also handily explains where Luna's bat-ponies came from).

The Princesses are Composite Characters of The Mane Six
. Celestia has Applejack's diligence, Rainbow Dash's mischievousness and loyalty and Pinkie's love of fun and parties. Luna has Fluttershy's social awkwardness, Twilight's by-the-book attitude (Traditional Royal Canterlot Voice, anyone? Trapped for a thousand years or not, there is no way Luna couldn't have noticed that nopony talks like that in Canterlot anymore) and Rarity's desire to be noticed and hamminess.

The Princesses are Fused Lifeforms
There are six Elements of Harmony, and three categories of pony, Unicorn, Pegasus, and Earth Pony. In the past, six ponies, two of each type, in a group similar to Twilight's Nakama banded together to defeat Dischord. But the amount of power it took caused the original six to fuse into two new beings, the alicorns. This is also why Luna and Celestia lost the connection to the Elements of Harmony.
 * It follows that the component ponies were the six Founders of Equestria from Hearth's Warming Eve, which is where the rulership of all came from.
 * So, which one is the leaders and which one is their advisers? For some reason I think Celestia's made from the less confrontational ones.
 * Celestia could use all the elements on her own against Nightmare Moon, if the legend was correct. So Celestia was still able to use the elements post Alicornization and Discord didn't seem surprised that she was an Alicorn.

The Princesses' parents are known as Queen Gaia and King Chronos.
Or as their children prefer to call them, Mother Nature and Father Time.

The Princesses were behind all the special effects for the Hearth's Warming Eve pageant
Especially the Wendigos. The only thing they didn't do was the Fire of Friendship, which they let Twilight do on her own.
 * And the Windigo roar at the end was their doing as well, to remind the Mane Cast of the lesson.

The Princesses came into being from ponykind's belief in them.
"Hearth's Warming Eve" shows the ponies of the newly founded Equestria putting up a flag depicting Celestia and Luna, even though the events described took place long before their reign. The Princesses - being a combination of unicorns, pegasi and earth ponies - were originally merely a symbol for the unity of the three pony races. As time went on, ponies formed a mythology around them and began to worship their image as deities. Eventually, the Princesses became Physical Gods due to the principle of Clap Your Hands If You Believe, saved Equestria from Discord and became its rulers.
 * Alternately, they came into being during Discord's reign. As things were hopeless, they turned to their deities, or possibly just refused to give up or the like. Their hope and unity united into a form they'd all agreed on to represent it, Celestia and Luna.

The sisters/princesses are the founders of Equestia
Note the flag that was planted in the Winter's Hearth play. The story either omitted or is partially corrupted. If ommission then the three "servants" succeeded their masters into rule and became the three Alicorns. If corruption, then there was ever only three ponys. The "rulers" of the play were given all the negative aspects and sympathetic "servants" were added for narrative purposes. Either way the results is the same. Celestia was the unicorn for near certain. Luna could go Earth or Pegasus. Cadance can't be placed directly till she appears in show. Maybe Celestia suggested relevant roles to the Mane Six based on this.
 * Celestia is at home with magic with the founding of the magic school, raising sun/moon alone, connecton to Twilight. Luna was likely the pegasus.
 * Luna as pegasus.
 * Personality wise she's a cross between Fluttershy (Shy ackwardness and the "YOU WILL LOVE ME!") and Rainbowdash (Pride and love of sports.
 * Luna announces the pony races apparently?).
 * Every season introduction to her has her coming from high down to low (moon to Ponyville, descending in a chariot).
 * Except in corrupted as Nightmare Moon she never really uses her magic, and appeared ackward with it so this is a mark against being the unicorn.
 * Zecora's story emphasizes Nightmoon flying.
 * When Twilight tries talking to Luna in front of her statue what does Luna do when going dramatic? Start flapping wings, hoving and creating a wing powered wind.
 * Appears way too out of touch to have been the earth pony.
 * Luna as earth
 * Except in corrupted as Nightmare Moon she never really uses her magic, and appeared ackward with it so this is a mark against being the unicorn.
 * She rode in on a chariot in Luna Eclipsed.
 * She may have Fluttershy like shyness and Maredowell ep like Rainbowdash breakdown, but that doesn't mean she was a pegasus. Leading to the breakdown could have been insecurities due to being "only" an earth pony.
 * General:
 * Luna knew Starswirl the Bearded, a magic using pony who was famous for his "animorphic spell". Animal morphing.
 * Actually, they said that Starswirl the Bearded was the father of the "amniomorphic" spell which apparently means bowl shaping.

The princess are the purified Windigos
The Windigos are beings that feed on disharmony to create snowstorms and coldness. Once the first founders of Equastria created the Hearth's Warming Eve. Ala Nightmare Moon, instead of destroying them, they were purified and became the princesses. If enough disharmony occurss between their subjects, they will revert back.

The princesses are descended from Princess Platinum
With her title, it just makes sense. Plus, the alchemical symbol for platinum is the sun symbol joined with the moon symbol. Perhaps Rainbow Dash was crossplaying and Commander Hurricane was male, and perhaps he and Princess Platinum came to more than simply tolerate each other, resulting in the two alicorns we've seen.
 * I think the wording in the play does confirm that Commander Hurricane was male, the other characters constantly refer to him as "he".
 * And yet Private Pansy really really really really really really really really dislike HER.
 * That could've been a slip up on Fluttershy's part. There's already speculation that half of how Private Pansy acts is due to Fluttershy's bad acting (starting with Rainbow Dash needing to prompt her first line).

The princesses were born to regular, mortal parents.
Something incredibly strange happened during the mother's pregnancy, a rare and magical event, that caused the fillies to be born as immortal Alicorns with god-like powers. They may have even had mortal siblings. This would explain why they have non-Alicorn relatives.

Luna and Celestia do have other Alicorn relatives.
Like Luna and Celestia, they have powerful magic that they use to control major natural events to help keep the planet and Equestria running. However, they have to live on all parts of the world to do so, and they can only get together on major events, which is why we rarely see them.

Celestia and Luna both have a a team of nine elite guards
They train the cadets and play war games in their spare time.
 * Nine each? Or nine for the both of them?

The title of "princess" is a case of Call a Rabbit a Smeerp
The role and functions of the position are equivalent to that of a queen (or co-queens, given we have two of them), but because this is not Earth, for whatever reason the position has the title of "princess."
 * Jossed by Queen Chrysalis. Maybe it is due to God Save Us From the Queen?

The Princesses' family/relationships
Luna and Celestia both have direct descendants living in Equestria that come from ancient affairs from thousands of years ago. "Taller" ponies are the result of Alicorn blood, and an Alicorn will rarely be born to parents with Alicorn heritage, e.g. Princess Cadence. Twilight and her brother are unique because their parents are both direct descendants of each princess- their father is related to Luna and their mother, Celestia. Shining Armor takes after Celestia and Twilight takes after Luna (Shining Armor is taller and lighter than his sister, and Twilight is smaller, darker, and more reclusive.)

Luna and Celestia are exactly equal in power.
Though their individual aptitudes likely vary. The end result of this is that in order for one of them to defeat the other, they require a boost from an outside power source. Luna tried fusing with the evil force that turned her into Nightmare Moon to overpower her sister, but was trumped by Celestia when she used the Elements of Harmony.

Celestia and Luna can spend only small amounts of time around each other.
They are the Sun and the Moon, the Day and the Night. Their meetings are but fleeting.

The Princesses imprisoned their divine predecessors and the Windigos and a host of other creatures in Tartarus
It's how that section of the Underworld was primarily used in the Greek myths.

Nightmare Moon
== If a Pegasus Unicorn Princess named Luna gets taken by the Dark Presence, and serves as its Avatar through Nightmare Moon, then you're in... Night Springs! == Think about it. The "dark magic" that had captured Luna (like, say, Barbara Jagger) possibly feeds from her Leitmotif: darkness and night. It seems more or less crucial for it, just like the DP (which is in fact a literal case).

In both universes, it's visible form consists of black/dark purple blobs. It can take a story's minor protagonist and make her disappear from the plot altogether, making her the MacGuffin (like Princess Celestia, or Alice Wake). Luna has the power to bring eternal night, and Nightmare Moon!Luna plans to do so (unlike Normal!Luna).

Nightmare Moon!Luna is able to a) manipulate wild animals' behaviours (through a possible array of factors, such as the thorn), b) destroy bridges (seriously, the DP always destroys bridges) through levitation, c) play mind-whammy on characters to make them budge (the trees with the scary faces), and most importantly d) can probably 'take' characters, can teleport them to the scene (like regular Taken (Nurse Birch), only this time it was done with the "Shadowbolts", who may or may not have been an actual stunt-flying group before being taken) and use them as marionettes (like Rose Marigold), and Luna can be 'exorcised' from her possession by an intensive and bright source of light and colour (Friendship-Magic, in this case. Perhaps a Flare Gun would have done it too).

That she didn't carry away any permanent damages of sanity after said possession can be explained by the fact that the DP only 'touched' her (just like Alan Wake), which means that it was able to control it's thoughts and perception, but did leave enough liberty for her to use her special ability, this being controlling the night-day cycle (as opposed to Wake's writing talent).

And hey - both scenarios take plays in tv shows ("Previously on Alan Wake / My Little Pony")!

What happened to Luna will happen to Celestia
Because, honestly, I so want to see her "Nightmare Sun" form.
 * "Hellfire Sun" would be a much cooler name. And she wants to scorch the land!
 * There is no cooler name for Evil Celestia than "Mourning Star" (not mine, made by an artist on Deviant ART). It's punny, it refers to Celestia's domain - the dawn, and it's evil to boot.

Princess Celestia wanted Nightmare Moon to escape
She's the one who stopped Twilight Sparkle from trying to prevent it, after all. If she was really the heartless tyrant some people seem to think she is, she'd have wanted to make sure her sister stayed locked up, not welcomed her back. My guess is that she sealed Luna away because without the sixth Element of Harmony, that's all she could do. After a thousand years of guilt and loneliness, she just wanted her sister back.
 * Luna may have been the missing Element of Harmony. So, if Nightmare Moon really was controlled by a seperate evil force, by taking Luna it rendered Celestia helpless to free her sister.
 * Partly confirmed in the return of harmony; it was confirmed that Luna was one of the original bearers of an elemment of harmony and Lauren Faust confirmed the Luna was possessed, to a degree. I created an elaboration on this guess into a full theory.

Nightmare Moon didn't escape at all.
Nightmare Moon had an awfully orderly escape. The stars converged exactly on schedule, at a preordained date and time, and in perfect symmetry, just as the prophecy foretold, and even foreillustrated. Now, what's a simpler explanation, that somepony had a precisely accurate psychic vision that rogue stars, acting of their own agency and for no apparent reason, would assist in the escape of a, ahem, lunatic, or that the pony who cast the spell just wrote down how it would end?

Celestia, who loved her sister dearly, would not imprison her forever. She cast the spell to seal Luna in the moon, and in doing so ordained the date, time, and mechanism of her release. Knowing Luna would not listen to her, she made plans to train a protégé who could wield the Elements of Harmony and knock some sense into her wayward sister, and wrote the books of "prophecy" to facilitate that end. In essence, 1000 years wasn't a prophecy -- it was simply Nightmare Moon's sentence.
 * To elaborate on this: the "prophecy" states that "the stars will aid in her [Nightmare Moon's] escape". Food for thought: Celestia is in control of both the day and the night sky. Read between the lines, and the prophecy becomes "Celestia will aid in her escape". Celestia wrote that "prophecy" herself.
 * She worded it like this for two reasons - first to avoid backlash from her subjects about releasing a dangerous maniac, and second her own trickster tendency to leave things unworded for others to figure.

Nightmare Moon is a fusion of Princess Celestia and Princess Luna.
This assumes that the tale (Mare in the Moon) was wrong to begin with, meaning that Luna doesn't become Nightmare Moon, and it was just a tale told by the winner of the fight. And the fight went like this: Princess Celestia was truly angry at Luna for something she did or tried to do, banishing her to/into the moon as punishment, possibly in a time Celestia was far less mature and understanding, and only had her power in her favor to prove herself right. Further proof is that Luna seemed to fear her, and was very sorry afterward; Celestia must have been a monster when mad, but Luna truly loves her big sister.

And as for how is Nightmare Moon a fusion of the sisters, it's all in Celestia's disappearance and reappearance times: she disappears after being a confirmed appearance at Ponyville for the celebration and right under their eyes, and reappears after the burst of rainbow and light caused by the elements, freeing the sun. Also, it's awfully convenient that Nightmare Moon put Twilight Sparkle and friends under the precise circumstances to produce the Elements of Harmony later on...
 * While that does address Celestia's disappearance and the tests, it should be noted that Nightmare Moon, iirc, mentions "her sister," rather than just "Celestia."
 * Maybe Nightmare Moon is a fusion of Celestia and Luna with Luna's personality being dominant. If Celestia's personality dominates, we get a completely different being: A merciless sun threatening to turn everything into desert. So essentially, when they both fuse, they don't produce a mix of both characters, but one is always dominant and it's aspect is amplified to it's extreme while the other is suppressed.
 * Celestia's appearance changed when she started ruling over both night and day. With Celestia banished, Nightmare Moon is ruling over both day and night. Perhaps Celestia, with her power stolen, looks more like Princess Luna does now? We've only seen them when they were each ruling only their own domain except in the stylized intro to the first episode.

Nightmare Moon is Derpy.
Luna is just a fall pony tricked into believing she did the deed. Ever notice how Derpy seems to show up everywhere? That's because she's watching, looking for weaknesses in the Elements of Harmony and preparing her next master plan.
 * That's why she dropped all that stuff at Twilight at one point in "Feeling Pinkie Keen". She wanted to destroy the sixth element.
 * If not Nightmare moon herself, definately, a henchman... hench pony.

Night Mare Moon's one thousand year imprisonment on the moon was a Karmic punishment.
When she first revealed her plans to make the night last forever, she said, "The moon is the only thing you'll see for a thousand years!"

Nightmare Moon's evil isn't gone or destroyed.
It was absorbed by the Mane Cast and it is corrupting them. Every time one of them feels extreme rage or acts crazy, it is because of the same power that was corrupting Luna. And this Dark Side is growing stronger...

Nightmare Moon was actually Princess Luna's nightmare (building on the idea, above, that the two are not the same pony). Also, who is Princess Celestia, anyway?
Luna was very jealous, but unable to bring herself to upset The Harmony to do anything about it. Denying her feelings, she pushed them down until they gave her bad dreams, and finally Nightmare became strong enough to take over. This essentially means Luna was sick, not evil, which agrees with the show's White and Grey Morality. When Luna said she was sorry, it wasn't for what Nightmare almost did, but that she hid her feelings and created her (although it hasn't been stated in the show, lets assume that she was getting therapy on the Moon instead of just locked in a closet).
 * Interestingly, if this guess is correct, then the Celestia we see may not be the real Celestia, either, but actually a daydream (a thankfully happy one).

Princess Luna was Nightmare Moon!
I know it sounds like a crazy conspiracy theory, but think of it, my fellow ponies! Celestia introduced her sister of which we have never heard before to us right after Nightmare Moon appeared, and then mysteriously vanished again. Isn't that a strange coincidence? And she gave her responsibility for the moon. The moon, don't you get it? I tell you, there's some crazy conspiracy going on behind our backs, fellow ponies! So, never stop asking questions! The truth is out there!
 * Er... is this a joke? Everyone already knows Nightmare Moon was Luna overtaken by her own jealousy, and what Twilight did with the Elements of Harmony was free her from what she had become.
 * Oh my... yes, this is a joke. Normally I'd say Don't Explain the Joke, but here we go. Of course, we viewers know that this is true, but this was supposed to be written not from the point of view of us viewers, but of some pony from Ponyville. Think of it that way: We viewers also know that none of the ponies from Ponyville even knew the name Nightmare Moon before the incident, and even Twilight was surprised to hear that Nightmare Moon was Celestia's sister (although she had just read it in a book one episode earlier). When Celestia and Luna return to Ponyville after the Nightmare Moon incident had been resolved, Celestia just introduces her sister to the other ponies without even mentioning that Luna was Nightmare Moon. She just says something like "This is my sister Luna". So some of the common ponies might come up with such weeeeird conspiracy theories... which, as we viewers know, happen to be the truth.
 * Even though Twilight Sparkle read that Nightmare Moon was the sister of the unicorn that raised the sun, she may very well not have known that Celestia was that unicorn. The book only said that the elder sister "took on responsibility for both sun and moon" and that harmony had been maintained in Equestria for generations. It never said that it was the same pony doing the maintaining.
 * Fair enough. Also, this sort of supports my overall point.
 * You're just being ridiculous. They don't even look alike. An entire crowd of ponies saw Nightmare Moon in Ponyville, and every single one of these eyewitnesses described Nightmare Moon as a large alicorn, about the same size as Her Divine Majesty, Princess Celestia, with a black coat and a mane of and tail of some sort of dark, shadowy aura. Princess Luna is only about half the size of her sister, and has a purple coat with a mane and tail of dark blue hair. Seriously, they look nothing alike.

Nightmare Moon is magically aged-up version of Luna.
Word of God states that Luna is not fully grown yet and that her adult form will look less like Nightmare Moon, which means that NMM is a warped version of her future self. When she tried to rebel, she thought that the ponies did not respect her and her night because she was the younger sister, so she used magic to make herself older. Unfortunately, the magic enhanced the negative aspects of her personality. Thus, the elements of harmony were needed to counteract the negative personality to bring her back to normal.

Nightmare Moon was an Eldritch Abomination born of Luna's jealousy.
The reason she could overwhelm Celestia was due to being possessed by a horror which seized her in a moment of weakness and made her sublimely powerful. Only the Elements of Harmony could seal the horror back up and free Luna. (Admittedly this is one I used for a fanfic...)
 * Word of God states that Luna was transformed into Nightmare Moon by "an outside force", partially confirming this.

At some point, we'll get to see Celestia's take on the Nightmare Moon form.
Apparently, Nightmare Moon was not Luna's true form, just an evilized version of her. We'll get to see a evilized version of Celestia, which will an appropriate evil name. (Like Hell Horse or something like that.) There are two ways this can go down:
 * We'll get a Halloween episode, where this will be her Halloween costume. She may try to scare the ponies with this idea.
 * There already is a Halloween episode, but none of that happened at all.
 * The same magic that corrupted Luna will hit Celestia, causing her to become more and more evil over time. The main cast will team up with Luna in an attempt to bring her back to the light.

Nightmare Moon was Princess Luna's Heartless.

 * I like this idea, but where is Luna's nobody? it is possible that it was destroyed by Celestia(who, for the sake of this theory working, only sealed Nightmare Moon because she couldn't bring herself to kill her own sister), who only did it because she knew nobodies don't have emotion, so it wasn't really her sister.

Nightmare Moon was banished, but Princess Luna went voluntarily.
Although Nightmare was dominant, Luna was still present. The banishment was more like a time out or a "go to your room" situation (Luna & Nightmare are both embodiments of the night and controlled the Moon, before Celestia took over). Nightmare was a vengeful aspect that wanted eternal night and thought nothing of the consequences, but Luna would have known what would happen if that were allowed and wasn't able to stop it. Celestia won by using the Elements of Harmony, but it was Luna that helped keep Nightmare on the Moon, wanting to keep her evil, dominant side from hurting anypony.

Nightmare Moon/Luna built a Lotus Eater Machine on the moon.
In an effort to keep her sanity during imprisonment, she created an illusionary paradise for herself on the moon. She had a small, but grand and majestic lunar kingdom, filled with subjects that deeply loved her and her night. The illusion's backstory for the subjects is that they were a resistance force that formed after Nightmare Moon/Luna's banishment, but were banished to the moon themselves and helped their princess build her new kingdom. Most of her years of imprisonment were spent there happily, and Nightmare Moon/Luna eventually came to believe it was real. However, it was eventually destroyed by the clashing personalities of Nightmare Moon and Luna. Sometimes one would be dominant over the other, and the paradise would alter a bit to make the dominant one happy. (For example, one day Nightmare Moon would be in control, and an illusionary Celestia would be captured and killed. The next day, Luna would be in control, and she and Celestia would spend the day together, having made up long ago.) The lack of continuity eventually caused the dream to collapse, leaving a heartbroken and furious Nightmare Moon alone for the rest of the sentence.

Princess Luna was possessed by some dark force, possibly another world.
When Luna saw noone was out enjoying the night, her negative feelings left her wide open to some dark spirit. It possessed her, transforming her into a warrior of darkness. It would explain the size difference between Nightmare and Luna.
 * Confirmed by Word of God.
 * That explains why Nightmare Moon was in firm possession of the Villain Ball for the second half of the pilot -- Luna was Fighting From the Inside, deliberately putting the ponies in situations where the one holding her Element of Harmony could show it (after the initial incident with Applejack and the cliff, which was just an accident).

Nightmare Moon is/was a Dark Passenger possessing Luna.

 * I am GOING to tell Fluttershy you stole my idea... In any case, as I have been told, confirmed by Word of God... at least the possessing part. The Dark Passenger bit is probably jossed.

Remember that bit with Nightmare Moon in the hourglass?
That was the result of Nightmare Moon testing the limits of her prison. As centuries went by and it weakened, Nightmare Moon was able to appear in Equestria for a few brief seconds in random places, unable to really do anything but look menacing (at first, though she later was able to influence Equestria in small ways) or be detected by most people. There were a few cases where she'd appear right behind other ponies, without them noticing anything but an odd chill in the air. Her reflection in the hourglass was one of her final such appearances, with Twilight and Spike too busy to notice anything.

Yes, I did just turn Nightmare Moon into the Slender Mane.

Nightmare Moon was incapable of removing the Elements Of Harmony from their resting place herself
This is why she just didn't head on ahead and destroy them, even though she knew their location. Either because she wasn't one of the proper weilders or maybe some spell put on them by her sister or the fact she was evil, Nightmare Moon was incapable of removing them from where they were kept. Thus why she kept trying to stop them from reaching them, if they couldn't reach them, they weren't a threat to her. She let them alone once they reached them so she could swoop in and grab them like she did, because once they were moved whatever protection was on them was broken. By that point, she decided to kill two birds with one stone and destroy the Elements and her enemies all at once.

Banishing Luna/Nightmare Moon to the moon was really the only option Celestia had.
In order for the Elements of Harmony to be used to cleanse someone, there has to be at least two wielders. This is because the elements embody The Power of Friendship, which has the power to heal in the MLP universe. Since Luna became Nightmare Moon, she lost her connection with the elements and the relationship between her and Celestia became disharmonious. Celestia was the only one left who could use the elements, so she obviously had to be the one to stop Nightmare Moon. However, as she was alone, she could not heal Nightmare Moon. Since she did not want to kill her sister or turn her into stone, Celestia sent her to the moon. While such an act restored the natural harmony of the world, it destroyed the bond between the sisters. Since Celestia used the elements to destroy a relationship instead of creating one, she too lost her connection with the elements. Therefore, Celestia had to find others to wield the elements in order to save her sister.

Nightmare Moon used to be Luna's playful persona.
Based on the leak info about Luna's episode, ponies' Holloween is called "Nightmare's Night" so it wouldn't be that hard to guess that Luna is a trickster and use to go out once a year to play tricks on ponies, kinda like a pony Loki. Even Nightmare Moon on the show seems to more Laughably Evil and an Evil Overlord most seem to think she is.

Nightmare Moon is still alive and well
It is quite clear from what we have seen of them that the Eo H are incapable of killing. Most likely, when NMM possessing Luna was blasted by them, it merely separated the two. Part of Season 2 will be about the Big Bad trying to bring her/it back to physical form.
 * Dunno, Nightmare Night implied that NMM and Luna were one and the same, save for her being out of her mind as the Nightmare.

Princess Luna and Nightmare Moon were originally two seperate entities, fused together by the Elements of Harmony
Word of God states that an outside force transformed Luna into Nightmare Moon, originaly, Nightmare Moon was an Eldritch Abomination that posessed Luna and used her body as a "host" for the Nightmare Moon entity. When Nightmare Moon was defeated, the Elements of Harmony did not destroy her, but more rather, fused her and Luna into one being. This explains why Luna always referred to herself as "we" in Luna Eclipsed, referring to both Luna and Nightmare Moon, this also explains why Luna sounded like two different people talking when she used the THE TRADITIONAL ROYAL CANTERLOT VOICE, because it was both Luna and Nightmare Moon speaking simultaneously.
 * Yes, because that certainly has to be the reason she uses "we", despite her explaining the use of Royal We in the episode.

Nightmare Moon's first attempt at eternal night lasted several months or even years.
Nightmare Night indicates that Nightmare Moon is remembered as a very influential and terrible figure in Equestrian history, even if the details are all jumbled. If Nightmare Moon's first attempt at eternal night (before Celestia banished her to the moon) only lasted the hours or days that her second attempt seemed to, then she wouldn't have been remembered nearly as much, and some ponies probably wouldn't even know her name. Rather, it was a very dark time in Equestria's history, with widespread famine and roving monsters of the night terrorizing Equestria's citizens. ""You ponies are just the most fun I've had in eons""
 * But wasn't Nightmare Moon banished to the moon the same year (1000 years ago) that Celestia and Luna imprisoned Discord? Moony's reign would have to have lasted the rest of that year, at most.
 * No, or at least there is no reason to think that. The show implies that Celestia and Luna reigned together for a long time after defeating Discord before Luna's rebellion.
 * They are rounding the numbers. While keeping count of Luna's banishment served a purpose, the imprisonment of Discord did not, so, say, Discord was taken care of 1179 years ago, that comes simply as "a thousand years" ago
 * Again, though, there is nothing in the show that states how much time passed between Celestia and Luna's defeat of Discord and Luna's rebellion against Celestia. It could easily have been many thousands of years, or even millions.
 * Though neither Celestia or Luna mention how long Discord was imprisoned for, Discord himself does.

- Discord

The stars that aided Nightmare Moon's escape were the other 4 Ancient Wielders of the Elements of Harmony.
I'll be honest here, I don't know where this would go, I'm just putting it here because Princesses are involved:// There are six elements. As far as we've seen, you need a specific team of six to use them. Celestia and Luna are both members of the old team. That leaves four slots open. How many stars were there? Four.// Since I'm here, I might as well put what I think the princess were, too. Celestia was wearing the Element of Magic Tiara in the Intro Story, so we know she's the Magic in the six. Luna was probably Loyalty, and I base this entirely on the fact that it took the entire team to catch Graynbow Dash in Return of Harmony Part 2. "You: But how the hay did they use it on NMM if she was a corrupt Luna?"

Simple, Luna was an unwilling pawn to the Nightmare Possessor, and fighting from within, trying desperately to stop Evil!herself from doing the evil things she do her true self shines through at the last minute, and she helps seal the beast, knowing full-well she'll be trapped with it.// Back to the original point: Celestia wants to save Luna, but the seal can only be broken by the Elements, one of which is trapped on the moon. So she hatches a Plan. The other Ancient Element Bearers, who are all mortal ponies, are transformed into the stars before they died, and when Celestia gave the word, they'd open the seal for Nightmare Moon, just in time for her plan to be put into action. Bing Bang Boom, Twilight and the others, Episodes 1 and 2. Luna freed, the other four get to chill for eternity in the sky playing the ultimate game of Eye-Spy, everypony lives happily ever after… until the Cutie Mark Crusaders released Discord.

The story of the Blanks has something to do with the creation of Nightmare moon

 * There really was a village in the everfree forest which killed all ponies who acquired "The mark", And are now cursed to be zombieponies forever. Well, suppose that a thousand years ago, A filly from Ponyville wanders in and discovers this village and it's secret. She is cornered and cries out for help, And Luna swoops in to save her. However, Luna is unable to defeat the zombie villagers entirely, She could not completely erase their evil. Though the village was destroyed and the Zombponies all slain, The Evil that brought it all about still lingered, and took root in Luna's growing feelings of envy towards her sister. In time, it grew, and Luna became Nightmare moon.

The entire "Nightmare Moon" scenario was the final scene of an elaborate LARP.
We've all seen that Celestia is a bit of a troll, right? (The episode with the teacup and the phoenix, anypony?) Assuming Luna has a similar sense of humor, her plot to create The Night That Never Ends was just another game to the two goddesses, with Nightmare Moon as Luna's Player Character. (Being immortal, a thousand years is probably the normal length for a game to them.) "I missed you so much, big sister" is just Physical God -speak for "Good Game". Of course, Even Jerkass Gods Have Standards, hence Celestia's treatment of Discord.

Nightmare Moon is a mask of Nyarlathotep
As in the Cthulhu guess below, except for the fact that NMM actually spoke to her victims. Furthermore, the assorted "tests" she put the Mane Cast through were his way of toying with them.

Nightmare Moon will be the main villain but...
Spend most of the season in her smoke form, unable to manifest physically. After she was beaten and purged from Luna, she's been searching for a way to get a new body and her revenge. Many of the bad events in the preceeding season will be revealed to be her doing as attempted revenge. Her plan will revolve around trying to create herself a body free of any good influence, and it will be revealed that Luna was holding her back previously, thus why she doesn't want a body with a pre-existing mind in it.
 * See: The Fanfic Past Sins

Nightmare Moon is Cthulu.
Locked some place cold and dark (Moon/Depths of Ocean), incredible powers, will escape when the stars are right, treated as myth by the local civilization (Ponyville/Everyone who hasn't been involved in anything) but known by foreigners (Zecora/cults) and people who read far too much (Twilight/?), it's form is sort of comprehensible but aspects are well beyond what people can grasp (Mane and fact she's an Alicorn / Giant octopus-dragon-man Starfish Alien).

Nightmare Moon secretly wanted to be defeated.

 * Had she won, she'd be ruling over a land slowly dying due to her eternal night, with her subjects cursing her name. When she lost, she was purged of the evil influence that got her banished in the first place, Easily Forgiven and reunited with her sister. Which outcome would you prefer?
 * When you get right down to it, Nightmare Moon's attempts to stop Twilight and her friends from reaching the Elements of Harmony weren't as dangerous as they could've been, or even well thought out. Trying to knock them over a cliff is dangerous, but not very when you consider that two of them could fly and carry the others to safety, and the manticore and the serpent weren't really trying to stop them at all. The faces on the trees were especially pathetic when you consider that they didn't really do anything other than look scary, and simply laughing at them made them disappear. The only really effective method was the Shadow Bolts, and they could be stopped simply by turning down their offer.
 * Isn't it convenient that each obstacle just happened to allow a particular pony to demonstrate the virtues they ended up representing? Perhaps that wasn't a coincidence.
 * If Nightmare Moon wanted to separate Twilight from her allies, why teleport her such a short distance away? Why not send her halfway around the world? It's almost like she wanted them to find her and trigger the spark, forming the sixth Element.

Nightmare Moon was Luna all along.
To be sure, it may have been a Luna who wasn't exactly in her right mind anymore thanks to whatever evil outside influence one wants to credit for it. But there was no demonic possession or anything of that sort going on, nor did Nightmare Moon spontaneously split off of the 'real' Luna as a separate personality; there was only one pony underneath the mask all the time. This actually explains Nightmare Moon's actions in the pilot just as well as any other theory -- not so much any sort of dedicated Evil Overlord or Omnicidal Maniac as simply a fed-up filly goddess who's finally managed to send her big sister to her room after being stuck in her own for a thousand years and who's not going to put up with anypony ignoring her anymore ever...

Nightmare Moon was caused by/actually one of the Changelings
Think about it. She and Queen Chrysalis (so named in the script by the writers) both share dragon eyes, are powerful enough to take down Celestia, of all ponies in a straight-up fight, and they're both shapeshifters. Heck, Nightmare Moon is like them in another way as well, given that the Changelings feed on love in truly horrible fashion and Nightmare Moon desired the love of her subjects so much that she nearly destroyed Equestria with The Night That Never Ends.
 * For that matter, there seem to be some similarities between the Changelings and the wendigos too, with both being emotion eating monsters who want to feed on the ponies.
 * Before it was revealed that there was an imposter I thought that Cadence was a victim of the same thing that originally corrupted Luna, back to try again with a different princess.
 * Maybe she was...

The Nightmare Night traditions are the result of legends about Nightmare Moon and the changelings being combined.
Originally, the heroic (if scary) Princess Luna would defend the ponies from disguised spirits that wanted to gobble them up, and was given treats as thanks. After Luna became Nightmare Moon, that changed to her eating anyone who couldn't disguise themselves, and being given tribute so that she wouldn't eat them anyway.

Princess Cadance
== Upcoming characters Princess Cadance and Shining Armor (featured on a display seen during a party at Hasbro's world headquarters)'s roles in the show ==
 * Shining Armor might be a new Love Interest for Rarity.
 * If so, it almost definitely won't last, since the display depicts Princess Cadance and Shining Armor's Royal Wedding.
 * Very much jossed.
 * The Season Finale will involve the mane cast attending their Royal Wedding. If a villain decides to interfere, it may end in a Ballroom Blitz.
 * If a certain rumor is true,.
 * Confirmed! Except for that last part.
 * Perhaps they're Celestia and Luna's parents.
 * Seeing as the display shows the mane six and Cutie Mark Crusaders attending their Royal Wedding, unlikely unless major Time Travel is involved.
 * Maybe there'll be a Love Triangle with Prince Blueblood.
 * Jossed, though that would have been hilarious.
 * Cadance is Princess Celestia's daughter. Father doesn't have to be an alicorn, or even be mentioned. This also explains why she's not been addressed as a ruler, she's not. She also looks at considerably younger than them, about as old as Luna was when she was freed and doesn't have the magic hair. Unless she was sealed away for 1000 years too, she's got to be younger.
 * The reason we have never heard of her until now is because the Princess Sisters have kept her a secret her whole life out of fear that something might happen to her. Eventually, Cadance gets tired of being sheltered and locked away in a castle and decides to escape. over her journey, she meets the Mane Six, some freaky monsters, and of course, Shining Armor. They fall in love, Celestia finds out, freaks out, but eventually realizes that her daughter is a fully-grown mare now and she needs to live her own life and all that good stuff. And we will cry.
 * Jossed. Cadence is Celestia's niece.
 * Princess Cadance is in charge of the Equestrian military. This would explain her name and how she met Shining Armor.
 * Actually, Shining Armor is the one in charge of the Canterlot Guard.
 * Princess Cadance is a disguise, for a villain aiming to usurp Celestia and Luna by playing the "long-lost eldest sister" card.
 * Princess Cadance is a disguise, for a villain aiming to usurp Celestia and Luna by playing the "long-lost eldest sister" card.

Princess Cadance is Celestia and Luna's...

 * Sister
 * Stepsister
 * "MOOOOOM! Celestia and Luna are making a solar eclipse in the backyard!"
 * Cousin
 * Aunt
 * Daughter (Of which Princess, you decide)
 * Her cutie mark's a sun partially hidden behind a cloud (maybe her dad was a pegasus).
 * Only in fanart. Her official cutie mark hasn't been revealed yet.
 * it has now
 * Brother
 * Niece
 * Confirmed for Celestia.
 * Which could mean that she's Celestia's niece in the same way that Blueblood is (i.e. they're actually fairly distantly related, but refer to her as their aunt for the sake of convenience).
 * Father's brother's cousin's nephew's former roommate

the power of love in equestria.
to me it seems logical that when shining armor and princess cadance get married the power of love will:
 * transform him into an alicorn.
 * render him immortal.
 * turn the princess mortal via turning her into another type of pony.
 * It doesn't do any of that, but it does

Princess Cadance rules some other area of the world.
Which is why we haven't seen her in Equestria yet.

Princess Cadance is in charge of some major phenomena.
Princess Celestia is in charge of rising the sun and Luna is in charge of bring the night. So it's possible that Princess Cadance is also in charge of some major natural event. Maybe she carves the land or controls the tides?
 * Maybe she controls Death
 * Or possibly Time.
 * Probably stars.
 * Essentially confirmed: in a world where friendship is magic, her specialty is The Power of Love. Let that sink in for a moment.

Cadance was one of the last Elements of Harmony
She's the only other allicorn we've met, so using what we know it's worth suggesting

Cadance's case is essentially the same as that of Cakes' children
She's neither ancient, immortal, nor closely related to Celestia and Luna.

She was born in modern times to a noble family that had alicorn ancestry, which manifested after some thousands of years. This is probably an extremely rare thing, but Pound and Pumpkin Cake prove that 1) it's possible for a pony to be born a different type than his/her parents, as long as they have an ancestor of that type somewhere in their family tree and 2) that ancestor may even be singular and very far removed. And Prince Blueblood proves that there are ponies alive in modern times with alicorn ancestors. Why shouldn't it work for them the same way?

Cadance doesn't have to be in charge of anything like the Sun and the Moon, or ruling any land. Having a lineage tracing back to alicorns (or Celestia and Luna's family) may be all that's required to grant a pony a title of prince/princess (again, Prince Blueblood).

Cadance leads to more Luna.
It might be the writers’ wish to make episodes focusing on the immortal elders of our heroines. However, as both the Mane 6 and the CMC are multiples of three, the royal duo wouldn’t fit the show’s formula. Cue princess Cadance, the third alicorn. Hopefully, she will provide a lot of character development to her peers, and we will love her for that. Her marriage with Shining Armor could also make her the “mother” to Luna’s “maiden” and Celestia’s “crone”.

Princess Cadance and Shining Armor are referenced in S 2 E 17, "Hearts and Hooves Day".
Sweetie Belle reads from the book Twilight gave her: "Apparently, some prince (Shining Armor) who long time ago whipped up this recipe and gave it to this princess (Princess Cadance) he liked. He meant it to be a love potion, but things didn't turn out so well. There's something here about a dragon (possibly skimming over "Draconequus"), the kingdom falling, chaos reigning (another possible reference to Discord)... Okay, apparently it was all because the prince and princess were so lost in each others eyes that they couldn't perform their royal duties."

Additionally, the illustrations in the book they read depict a male pony (with no horn, however), and a female pony with a horn and wings. If this story is a reference to past events, Princess Cadance and Shining Armor may be historical characters.

Cadance is like the cupid of Equestria.
Her special talent is to help creatures find their special someone, or at least help find friends. Indicated by her heart-shaped cutie mark. The alicorn in the book was her, and the love potion was part of an experiment that went wrong.
 * Confirmed! It's unclear whether it's her duty per se, but her special talent is definitely Love.
 * Perhaps her talent is reinforcing bonds between ponies--she might be reminding the arguing couple, or  of their love for each other, pushing that bond and love to the forefront of their mind, so much so that it drowns out all else. As she got older, she learned to do it with more finesse so that it doesn't love-brain-blast whoever she uses her spell on.

Cadance's appearance will offer some explanation to alicorns in Equestria.
There are a LOT of questions to alicorns in the MLP world. Why are there so few? Where do they come from? Are they a special, separate species that happens to be very rare, or do they come into being some other way? Why are they so powerful? Are they truly immortal, or do they just have long lives? When Princess Cadance shows up, at least one question will be answered, or at least, get an implied answer.
 * Doesn't seem like it.

Cadance was specifically made into an alicorn for specifically for toy purposes.
After redesigning Princess Celestia to actually be white rather then pink, they were left with hundreds of perfectly fine, but inaccurate toys. So they decided to make an upcoming character an alicorn to sell these toys.
 * Partly Confirmed via Word of God; Lauren Faust was quoted that she was heavily involved in the wedding arc, and it was a surprise to her that Cadance was a winged pegasus. Unclear on what spured what though, maybe Cadance's promotion was a mandate from up top, and we got a white Celestia as a consolation prize?

Cadance is Evil.
Since we would all feel pretty stupid if it turned out she was an antagonist, and we didn't have it in Wild Mass Guessing somewhere.
 * It has been said before, yes, but that eyeliner of hers seems like a big clue in and on itself.

Cadance had numerous husbands.
Being immortal, she knew she was doomed to mourn them all eventually. Yet it has never stopped her from seeking love. After one of her relationships ended in tragedy because of a potion, she did try to be more like Celestia and Luna, but quickly found out that she couldn’t. Her defining talent is to help others find their very special somepony, who sometimes turns out to be herself. Family-wise, she is essentially the Gytha Ogg of Equestria, having great-grand-children in every corner of the principality. This, alas, includes Blueblood.

Cadance wasn't born an alicorn.
She started off as a pegasus, a minor noble. She won't discover she's a princess until the events of her debut episode. She'll come down with a case of adult-onset unicorn horn.
 * Perhaps her alicorn-ness is due to an artifact/spell/plastic surgery.
 * It was a gift from Princess Luna, a wedding present granting her her dream of being an alicorn, even if only temporarily.
 * Flashbacks show she was an alicorn even when she was young, so this is most likely jossed.

Cadance's dark secret is that...

 * She is at least part draconequus. John de Lancie said he had another MLP coming out, and someone on Equestria Daily pointed out that both discord and cadence are musical terms. Cadance may be Discord's daughter or something.
 * She gave Shining Armor a love potion. This episode might echo the backstory of Hearts and Hooves Day. It would also explain Shining's weird eyes in the trailer.
 * She's the pony of death.
 * She's not Celestia's niece... she's the daughter of Celestia and Discord, who used to date before the whole "raining chaos" thing.
 * She's the daughter of Luna and Discord. The preview only said that she was Celestia's niece.
 * She's a robot... who will go on a destructive rampage on said discovery of being a robot.
 * She has an overdue book. Both she and Twilight consider this to be a huge deal. All the events in the trailer are shenanigans that result from their attempts to find the misplaced book without anypony finding out. This eventually leads to Discord's escape.
 * She made a Deal With Discord to become an alicorn. Now, Discord is coming to collect his due.
 * She's a defective clone of Luna.
 * She's actually Celestia's Nephew
 * she's a mtf transgender.
 * She's (possibly a recently gender-switched) Prince Blueblood in disguise. Or maybe 'Blueblood' was her disguise all along.
 * She's really 3 fillies in an alicorn suit
 * CUTIE MARK CRUSADERS PRINCESS IMPERSONATORS YAY!!!
 * SHE'S THE GREAT AND POWERFUL TRIXIE
 * She's Discord himself
 * an evil goddess bitch using twi's bro to get close to twi so she can destroy the elements of harmony
 * She's pregnant, and Shining Armor is the daddy. The wedding is a shotgun wedding.
 * She only appears just now in-universe because she was still mentally recovering from the EXTREMELY messy divorce she had with Blueblood's father, as well as the shame from having her latest son, Blueblood, turning out to be just like all the other stuck up nobility in Canterlot, only worse. She's pretty proud of her daughter Fleur though.
 * She is a Spanish princess.
 * she was the one that send the friendship letters to twilight during discord's regin. Twilight only considered it dark because she doesn't like her for one reason or other.
 * She's not an alicorn at all! She's just a pegasus with a fake horn!
 * She's the alicorn goddess of stars, and she released Nightmare Moon.
 * She has tail extensions.
 * She's an evil enchantress
 * She does evil dances
 * And if you look deep in her eyes
 * She'll put you in trances
 * Then what will she do?
 * She'll mix up an evil brew
 * Then she'll gobble you up
 * In a big tasty stew
 * Soooo.... Watch out!
 * Upon the episode actually coming out, it's amazing how accurate this song is.
 * Well, it isn't like is an obvious guess.
 * The truth is that  However some of the theories here are still priceless.

We already met two of Cadance's kids.
They are Prince Blueblood and Fleur de Lise, Fancy Pants' trophy wife. They aren't at the wedding because Cadance specifically did not want the former to attend while Fleur was busy with something else and Cadance was respecting her wishes.
 * Cadence is shown to be only a little older than Twilight, and about the same age as Shining Armor. Blueblood and Fleur de Lise would be far too old to be her children.
 * Though that could be a result of Alicorn immortality.

Cadance's secret is that she's really, really, really stupid.
Her real name is Princess Namby Pamby. The dragons confused Celestia for her because she looks like pink!Celestia. She's dumb enough to have been held back in elementary school all this time.
 * Jossed.

Cadance is Prince Blueblood's sister
Because we already have the theory she's his mother.
 * Cadence is closer related to Celestia than Blueblood.
 * Since when? The viewers haven't been given detailed genealogical information on either of them.

Cadance is the Princess Goddess in charge of The Fine Arts
Because her name sounds a lot like Cadence- a sequence of notes or chords comprising the close of a musical phrase.
 * If anything, she must be the Princess Goddess in charge of Love.

Cadance will be a Parody Sue
Because it would be funny.
 * Seems jossed.

Cadance can alter her size at will
In the flashback to Twilight's foalhood she was shown to be barely bigger than the present day Mane cast, suggesting that she's not much older than them. But in "Luna Eclipsed" we saw Luna grow a foot taller and then back to normal as part of her Nightmare Moon disguise. She's probably hundreds, if not thousands, of years old and makes herself look older or younger depending on the situation, i.e. for babysitting it would be most appropriate for her to look like a teenager.

Cadance reincarnates every generation as opposed to the immortality of the Royal Pony Sisters.
Assuming Winged Unicorns are some sort of divinity/royalty whatnot of the world, it makes sense that Celestia and Luna are immortal and divine at some level, as they embody the sun and the moon and those aren't going anywhere anytime soon. Cadance could be the embodiment of equinity (humanity) itself, and instead of distancing herself from her little ponies, Cadance lives a mortal lifespan as a mortal pony, then is reborn a new mare, similar to the Dalai Lama or the Avatar of Avatar: The Last Airbender, with her Avatar State being composed of The Power of Love. This one is relatively young, and one of her older selves got in a little trouble with a love potion once.

The Want-It-Need-It spell was Cadance's Magic
Twilight is able to copy spells from exposure, and twisted Cadance's love magic to apply to an object with an indeterminate number of targets.

Princess Cadence is an Assassin.

 * You think about it, she DOES have an Italian-sounding name (full name: Princess Mi Amore de Candenza), like another Assassin we all know and love, and considering how much Assassin's Creed fanart is out there and how often the cast and crew give nods to the fandom, we might see some Robed Ponies out there.... and I'll be darned if one of them isn't Princess Cadence. If it weren't for the fact that Cadence's dark secret is that, this probably would've ended up being under the WMG which speculates at what her dark secret is.

Princess Cadance is also adept at increasing others' friendship and storgé.
Agape is an act of the will, so she's more limited in that department.

Princess Cadence will become far more powerful when she gets older
Celestia and Luna are very powerful, but they're adult Alicorns. Cadence is only a little older than Twilight, her full power hasn't manifested yet. When she does get to her full strength, she'll be extremely powerful because she represents Love, which is a close cousin of The Power of Friendship, the most powerful magic in existence (technically speaking, Friendship is Platonic Love, so it might fall under her power as well).

Cadence's faux pas in "A Canterlot Wedding - Part 1" were mostly Twilight being an Unreliable Narrator.
Twilight made up her mind that Cadence wasn't right for her brother as soon as she brushed off their Secret Handshake and was generally standoffish towards her at their first meeting. This tainted all of their subsequent interactions, and Twilight interpreted Cadence's behaviour in the worst possible way. Nopony else agreed with her assessment because Cadence's behaviour was actually fairly reasonable for a bride in her position, except for the mind control spell. Twilight was right, of course, but for the wrong reasons.

Cadence is not a 'true' Alicorn.
Rather, she is a semi-logical extension of Pumpkin and Pound cake's weird genetics. Where they got their unicorn/pegasus traits from old ancestors, Cadence got her wings from Celestia; however, unlike Celestia and Luna, who are combinations of all three races, Cadence is more like a Unicorn with wings attached: she doesn't seem to have the strength of an Earth Pony, and the only time she uses her wings is to glide when she and Twilight are, so it's possible she can't actually fly on her wings, only glide. Of course, she's still a very powerful unicorn, and descended from the Princesses, hence her title. She also isn't a goddess, just a very powerful unicorn whose special talent is something related to love.

Cadence being Twilight's foal-sitter was another of Celestia's tests.
In one Word of God Lauren told us that not every unicorn got the egg test, and that Celestia, suspecting that Twilight was the Element of Magic, set it up beforehand. Similarly, Celestia arranged for Cadence to keep an eye on Twilight before then, both to keep an eye on her magic and personality to see how likely that was.

Star Dreams is the daughter of Shining Armor and Cadence
Shining Armor is white with a blue mane while Cadence is pink with some pink in her mane. To put them together and get a light pink/purple unicorn filly with a blue and pink mane seems about correct.

Cadance's Bridezilla behavior in "A Canterlot Wedding, Part 1" was intended to make her come off as a Parody Sue Jerk Sue.
She randomly shows up in the story with close ties to one of the main characters, insists on being called by an impressive but overly-complicated name (Mi Amore Cadenza), she's suddenly the center of attention as soon as she shows up, and almost everyone loves her and fawns over her despite how mean and obnoxious she's being.

Princess Cadence is...simply the daughter of a brother or sister of Celestia and Luna whom we haven't seen yet.
There will be some fans that consider me a heretic for even suggesting this, but it is the least forced explanation for why she's considered Celestia's niece.

Cadence was originally intended to be the daughter of Fancy Pants and Fleur de Lis.
This one is based on Word of Dante stating that she turned into a winged unicorn during development, the ceremony being pretty clearly a unicorn wedding (rings and all), and the fact that while we see no other winged unicorns who could plausibly be her parents show up for her wedding, Fancy Pants does make a small but noticeable background appearance similar to that of Twilight's and Shining Armor's parents. (Note that even if this should be confirmed, whether these two retain any closer relationship to alicorn-Cadence in the finished episode would be at best up in the air barring Word of God or future developments.)

Queens are evil.
Girls thinking this is so is the official Hasbro reason why Celestia isn't a queen. Maybe that's canon in-universe as well: a past queen was so wicked that all future female rulers avoid the title.
 * Only one Queen has been seen in the series thus far, and she's Disney-level evil.

Queen is a reserved title, held vacant for a reason.
Like with Kim Il Sung in North Korea, a now-dead Queen holds the title officially, and no other Alicorn is allowed to be the true Queen.

Celestia and Luna have a living parent.
Living in the sense that an immortal magical being is truly alive. The King or Queen (or both) are still around in some form, but no longer physically present in Equestria. So, their daughters can't be queen.

To that end: the God-Empress of Ponykind is entombed within the Golden Stall. Her power preserves Equestria. To suggest otherwise is unfriendly, and also heresy.

There is no queen because Equestria is undergoing a massive Succession Crisis
There are in fact many Alicorns, and when the Celestia and Luna's parents died it kicked off a war that is still going on after thousands of years.
 * Or the situation with Nightmare Moon was over succession and over the following millennium Celestia started to regret banishing her little sister and told people to call her "Princess" instead of "Queen".

Celestia is the Queen.
Note that she is addressed interchangeably as "Your Highness," the vocative form of princess, and "Your Majesty," the vocative form of queen. Note also that there are constant references to the "royal" palace, the "royal" gardens, etc., not the "princely" palace or "princely" gardens. Clearly, like most monarchs, Celestia holds multiple titles. For comparison, consider that Queen Elizabeth II of Great Britain also holds among her many titles "Princess of Wales," although, as per ancient tradition, she permits her son and heir Charles to use the title Prince of Wales; she is also Empress of India, incidentally, but she customarily is referred to as the Queen. Celestia is both Queen and Princess, but customarily uses "Princess" so as not to appear too distant or removed from her subjects, an issue which obviously troubles her enough as it is. Luna, as the foalless Celestia's legal heir, is permitted to use the title of princess in much the same way as Charles is allowed to use the title of Prince of Wales.
 * Supported in "Super Speedy Cider Squeezy 6000" when Flim refers to the "entire kingdom of Canterlot." That suggests that Canterlot is a kingdom, of which Celestia is the queen, while she holds the title of princess over some other holdings. She probably also is the empress of Equestria. Most likely, however, she is called princess most of the time because the bulk of her dominions are principalities. So her full list of titles is probably something like Empress of Equestria, Queen of Canterlot, Princess of Cloudsdale, Princess of Manehattan, Princess of Fillydelphia, Princess of Trottingham, Princess of Hoofington, Princess of Ponyville, etc., etc....
 * The reason she goes by "princess" most of the time is so that she can seem more friendly and approachable to her subjects

Prince Léon, Celestia's nephew from that french comic, is much like Chibiterasu.
That is, there was a huge shifting of the balance between good and evil (perhaps the confrontations between Nightmare Moon or Discord). Apparently, him coming into existence was one of the results.

The Princess mentioned in Hearts and Hooves day was another alicorn.
The status of alicorns in the MLP universe is unclear, it's never stated that Luna and Celestia are the only ones. If Princess Cadance gets introduced in a future episode, it will confirm that there may be more than two.
 * Confirmed that Cadance will make an appearance in the show, proving that Celestia and Luna are not the only alicorns in the MLP universe. Perhaps the episode will explain the alicorn situation?

There are actually several alicorns, each controlling a different force of nature.
Just as Celestia and Luna control the sun and the moon, there are other alicorns controlling different forces of nature. One controls the tides, another controls volcanoes, another controls the moving of continents and the formation of mountains, another controls forest growth, etc. The reason that we've only seen Luna and Celestia so far is because, unlike the other alicorns, they can control their force from anywhere in Equestria. A sea-controlling alicorn must stay by the sea, preventing her from moving far.
 * My guess there used to be total of six alicorns. Celestia controlled sun, Luna controlled the moon, there was one that controlled animals, another controlled the seas, and another the weather. The final allicorn was a prince named Albern who represent courage in battle who was vital in the defeat of Discord and was likely a lover of Luna.

None of the Alicorns are related by blood
As they are so rare, any time an Alicorn is born, it is taken by the royal family and raised in the palace. Why? Because like how a baby unicorn has little control over it’s telekinetic magic, an Alicorn has little control over it’s [b]EARTH-SHATTERING[/b] magic. It needs to be raised away from the rest of ponykind, first off to control it, secondly to avoid forming emotional bonds with it’s still-mortal parents and siblings, and finally, to figure groom it as a possible heir to the throne. Celestia and Luna are considered “sisters” because Luna was found when Celestia was still a foal. When the two of them matured, all subsequent alicorns viewed the pair as adoptive aunts.
 * Alicorns are allowed to marry, but there is a good chance that they are sterile. Prince Bloodblood comes from Celestia’s biological family (they were all unicorns)

Prince Blueblood gets his title from the old Unicornian royalty, not his relation to Celestia
One branch of the family produced the princesses, who are...ahem...horses of a different color. Another produced Blueblood (and possibly Cadance, who got a lucky roll of the heredity dice and has been a cerapter from birth).
 * Which brings us to...

Blueblood is Princess Cadence's brother or cousin
And he acts as suck-up as he does to compensate for his lack of wings, as being a Unicorn Pegasus is the defining trait of royalty - lacking half of that defining trait, he compensates (probably subconsciously and due to the inevitable low self-esteem) by acting as snobby and "royal" as possible.
 * That actually makes a lot of sense.

Most winged unicorns aren't any more powerful than other unicorns, pegasi, or earth ponies.
Though having access to all three types of powers probably results in beneficial synergy. Cadence did not display power above the average unicorn or pegasus, though her special spell is extremely potent in the correct circumstances.

Winged Unicorns will always become royalty, or otherwise important.
In a fashion similar to diamond trolls from Discworld, they are too important on a symbolic level to fade into the background, regardless of how powerful they are.

Equestria has a lot of Vestigial Royalty
Celestia lets incorporated nations and peoples retain their titles, resulting in loads of impressive sounding but ultimately meaningless titles.

Alicorns are chosen, rather than born
Sufficiently badass ponies are chosen by some sort of deity or magical force to become alicorns. Celestia and Luna ascended after defeating Discord. Cadence, on the other hand, may be ex-military. I just pulled that last one out of nowhere.

Cadence, Blueblood and any other 'nieces and/or nephews' are only related via adoption
When Celestia and Luna were rising into power during their fight against Discord, various leaders with titles aligned themselves with the two alicorns. Once Discord was defeated and modern Equestria was founded, Celestia and Luna adopted the ponies that helped them into their family.