"Oh, no — another fan with ideas..."—Sokka's Actor, Avatar: The Last Airbender.
Ultimately, Canon is much smaller than the people who throw the term around like to think it is. Canon is limited to that which has actually been described in the source material. Especially in groups of writers, it boils down to what the writers specifically need to worry about for the purposes of the ongoing plot.
Fanon is the set of theories based on that material which, while they generally seem to be the "obvious" or "only" interpretation of canonical fact, are not actually part of the canon. Occasionally, the explanation seems good enough to just be "common sense." The salient point to remember is that when someone shouts, "That episode was terrible because it violates canon!", they are very often totally incorrect.
Fanon fills in holes that the writers may have deliberately left in order to have fodder for later stories. In addition to arising from a point of vagueness in the canon, Fanon can come into existence as a fact gained from a popular but non-canonical source, or taken from a different Adaptation. Because many fans mistake their own Fanon for actual Canon, they tend to get riled up when a new fact is introduced which does not literally contradict anything canonical, but invalidates what were formerly the most obvious assumptions. Many examples of Retcon and Continuity Drift that are imagined to be violations of Canon really only explicitly contradict Fanon.
Since many creators in the aftermarket series universe are fans, Fanon often shows up there, and if those creators in turn start writing for the actual show, Fanon may actually become Canon. Alternatively, you just have Memetic Mutation within the fandom.
Fanon often also refers to the body of information provided by otherwise-official sources. Television and movie scripts are a continuing source of fanon material — Captain James Kirk, for example, had a middle initial ("T.")...but his actual middle name ("Tiberius") was originally revealed in an episode of the Trek animated series; since that show's canonicity is debatable, it was considered "fanon" until revealed canonically in the 6th Star Trek movie. Note that this usage blurs the line between fanon and deuterocanon, though.
Warning: Fanon and accusations of Fanon are a classic Internet Backdraft, with the accusation commonly leveled by fans that have a different interpretation of the material — even when their theory is just as vulnerable to Schrodinger's Gun.
Compare Broad Strokes, where the events of a story are referenced in passing without taking everything said and done as having "officially" happened. If the fanon was repeatedly hinted at by writers until it became fanon, but never actually confirmed in canon, it's Writer Induced Fanon. See also Fandom Specific Plot. Not to be confused with this Fanon or the Pope's robe.
- Anime and Manga
- Film - Live Action
- Live Action TV
- Video Games
- Web Comics
- Western Animation
- The Lion King.
- Many fans think Simba and Nala had a son, named Kopa, who was either then killed by Zira, or otherwise died, or is at least gone somewhere awaiting a fanfic plot that re-unites him with his lost family. A confusing example, as Kopa was a character in the semi- or non-canonical storybook The Lion King: Six New Adventures published between the 2 movies, but was then never mentioned in the 2nd film. This has been speculated to be due to grief or something, but it's more likely that he was just Ret Conned out of existence. It's thought that he, and not Kiara, was the cub pictured at the end of the original movie. It's also due to the cub's coloration at the end of the first movie (resembling Simba's at the beginning and all the lionesses are a lighter color).
- It's required in fanfics that if Kiara and Kovu have a son, he be named Tanabi.
- Considering the other Lion King movies aren't officially in the Disney Animated Canon (which doesn't necessarily mean anything to the Lion King universe), some fans just dismiss Kiara's existence completely and replace her with Kopa.
- Nala was meant to have a younger brother named Mheetu but he was taken out of the final product. Fans often include him in fan-works, either as her unseen brother or her dead sibling.
- The Princess and the Frog
- Despite (or because of) the fact he's from a fictional country, many fans still throw Fan Wank fits over Prince Naveen's "real" ethnicity — namely, whether he was closer to Mediterranean or Middle Eastern and/or Indian on the sliding scale of Ambiguous Brownness. That is until some people picked up on a throwaway line in "Friends on the Other Side" ("You come from two long lines of royalty") and ran with it. The final conclusion? He's all of them.
- There's also the matter of Tiana being outbid on the restaurant. It has generally become accepted amongst fans that the "other buyer" story was orchestrated by the mill's owners, either with them outright lying or with them getting someone to outbid Tiana's offer. While this does explain why Tiana conveniently is outbid the very day she tells the owners she's ready to purchase the place (and why no "other bidder" gets angry when Tiana has Louis scare them into selling), there's no official explanation for it in-movie.
- How to Train Your Dragon fan fiction seems to have reached the consensus that a) Toothless is the Last of His Kind and b) Toothless is responsible for accidentally severing Hiccup's foot.
- Toothless's Last of His Kind status has pretty much been confirmed by both How to Train Your Dragon 2 and advance information on How to Train Your Dragon 3.
- The Sword in the Stone. Many fans were so saddened by the heartbroken girl squirrel that several fanfics have emerged in which Merlin turns her into a human so she and Arthur can be together. Popular fan names include Hazel and Gwen. These stories are accepted by many as legitimate Canon just because they want so much for the poor little thing to have a happy ending.
- Toy Story
- The fandom has it that (SPOILERED for Rule 34) Woody's pull-string is a sort of erogenous zone, since he lacks the necessary parts for sex. And by extension, Jessie as well, as she has a pull-string too.
- Also, unless you ship Buzz/Woody or Woody/Dolly or Woody/Jessie, the figurine of Bo Peep was bought by Bonnie or her mother and is reunited with Woody.
- Many fans theorize that Andy's mom was Jessie's original owner, Emily. There's no confirmation on this, but it would explain why she let her son keep a pair of toys that all but materialized in her house out of nowhere.
- In Cars fan fiction, the name of the agency Finn and Holley work for is called C.H.R.O.M.E, which is derived from the video game adaption, but the agency's name is never actually stated in the film itself.
- The Ordial Plane is a concept that turns up very frequently in Planescape fan work, based on the assumption that the Astral and Ethereal Planes should have a third counterpart in accordance to the Rule of Three and which would complete the circle between the Inner, Outer, and Material Planes.
- Warhammer Fantasy deliberately leaves a lot up in the air concerning some of its mysteries, but a great deal of fanon proclaims definitive answers to these questions. One common example is the nature of the Bretonnian goddess: many fans declare that she is merely a deception by the Wood Elves to be holy writ, but this is only one of several possibilities vaguely hinted at by the actual canon.
- The above applies to Warhammer's sister game Warhammer 40000 as well. There's even some (semi-) Ascended Fanon.
- There are fans who insist that the Eldar created the Tau Ethereals, based on some vague hints in the Xenology book, which had a decidedly Unreliable Narrator (and the narrator himself dismissed the idea out of hand.)
- Also, most issues related to memes that aren't pure gameplay bugs (and some of those, too) seem to have some fanon to them. E.g. it's canon that Ultramarines are generally by-the-book sort (the book in question happens to be written by their Primarch), think themselves "first among the equals" and try to act accordingly, though they do use unconventional moves if necessary. Fans tend to extend this toward more obsessive or stuffed and arrogant "Ultrasmurfs" a la Matt "Spiritual Liege" Ward. Like this.
- The fandom seem to be easily fascinated... in various ways... with the concept of death cults - and loves to build upon this.
Anonymous 1: I love the emerging theme that, given any excuse at all, a crew will form a death cult. It's not a question of IF a ship will have a death cult, it's a question of how long until it DOES get one.
Anonymous 2: Of course they will, it's the Imperium. The real questions are whether or not it's heretical and whether or not it's useful.
- Then there was that whole deal with the Squats...
- Dreadnoughts theoretically are "revered", but there are... inevitable issues. Fanon has the quintessential "Grandpa Dreadnought", but existing ones tend to be interpreted as having various "Cranky Grandpa" personalities too. And they fit for this, though it's probably would be more accurate to assume they are trying to stay more than a part of a ritual and have little time for chat.
- Bjorn the Fell Handed of the Space Wolves. He is the oldest human alive in the Imperium, not counting the Emperor who was immortal to begin with. He evidently has superhumanly keen instincts, and even back then was a berserker who can keep his head cool - which is to say, one of the most dangerous things around. Fanon: Bjorn is awakened every century or so, to let the Space Wolves learn from his ancient wisdom - that is, ask him to tell one of the few stories about Leman Russ - and it's back to sleep... and he is understandably fed up with this.
- Black Templars have Tankred (from Damnation Crusade, comic about Black Templars), who supposedly is in it for the bitches (and booze). Also, Tankred endures. One, he refers to himself in third person. Two...
- There's a scanned page where the Dreadnought wakes up and voices his complaint - though it's not clear whether this was plain truth, a joke to brush away other concerns, a way to guilt-trip his Battle-Brother, or all of the above. It's not even that his revelation only barely stays within Double Entendre territory, it's that Dreadnoughts have No Indoor Voice (and why would they?) - as far as he knows, a whole Company may hear this:
- In Neopets, a popular fanmade Neopian Times piece ("Poor Dr_Death") managed to define everything pertaining to the owners of the pound/adoption center. Most notably, the anonymous Uni was given a name, and nobody has found cause to dispute Dr_Death's characterization as a lovable Deadpan Snarker. (At least, not until his official appearance suddenly became much Lighter and Softer with the rest of the website, but that's another issue.)
- It's become Fanon in the shipping community that Ask That Guy is The Nostalgia Critic's twin brother and that he lives in his house. And that they're very close.
- And that The Other Guy is the older, slightly saner brother that the Critic goes to when he needs to lick his wounds.
- Again with them, any fic that takes place when they were younger calls them Doug and Guy. For common sense purposes really, it'd be silly for their parents to call them "Critic" and "Ask That Guy".
- Spurred by Ask That Guy's love of his pipe and Doug's penchant for blowjob jokes, Critic being really good at oral has become almost a meme in fics.
- Also that Dr. Insano's son is named "SOI" (Son Of Insano). And he goes to school.
- After Kickassia was finished, it's usually accepted that the Critic was exiled to a hotel room and the others had fun on his tab. What tends to happen next is, unfortunately, wishful thinking.
- Humanized versions of the Happy Tree Friends characters almost always follow a certain design.
- Space Marine so horribly mangled that replacing parts with bionics doesn't cut it, and as such buried in a life support sarcophagus, which is then stuck in a Mini-Mecha, and have to be set on hibernation most of the time