Butterfly of Transformation

"If nothing ever changed, there would be no butterflies."

—Anonymous

Butterflies -- the products of a process by which a crawling worm becomes a beautiful flying creature -- have long been common and obvious symbols of metamorphosis and rebirth in folklore and fiction; since that's how they grow in nature. In the most extreme cases they can be emblematic of death and subsequent rebirth, but they can also be used to represent all manner of other less literal or drastic transformations. And since butterflies have an ethereal quality to them in addition to the power of flight, they are frequently used as symbols of a Heel Face Turn. (Possibly as more worldly stand-ins for angels, who are also reputedly ethereal and capable of flight.)

A frequent variation is to start with caterpillars and use the entire transformation process, rather than just the end result, as the symbol. Moths are also used, but in these cases you won't get Pretty Butterflies as a side benefit. Further, moths are nocturnal (fictionally, at least), and you know what that means.

On the other hand, sometimes butterflies in a work are just pretty for the sake of being pretty, and the only symbolism you get out of them is that which you put into them in the first place.

This trope is the Super Trope of Butterfly of Death and Rebirth. Unrelated to the Butterfly of Doom.

Examples of Butterfly of Transformation include:

Anime and Manga

In Sailor Moon's third season, a butterfly motif is heavily featured in the titular heroine's Transformation Sequence in "Crisis Make Up!" were she transition from Sailor Moon into Super Sailor Moon. This not just touches on this but also on the Butterfly of Death and Rebirth, which is used elsewhere, as in using the Sailor Soldiers' power themselves instead of using the usual Holy Grail Transformation Trinket, which was in the hands of the foes.

Comic Books

In the The Inhumans, one of the Inhumans was petitioning to be re-exposed to the Terrigen Mists, because she's hoping for a better genetic draw since she's ugly. She transforms into a butterfly woman.
Psylocke of the X-Men, her telepathy's butterfly motifs symbolize this. Fitting, since her character has undergone tremendous changes, becoming Captain Britain, gaining new psychic powers, and switching bodies.
In 52, the Famous Last Words of Vic Sage invokes butterfly symbolism. This was addressed to Renee and also foreshadows her transforming into the new Question.

Film

In The Silence of the Lambs, Jame Gumb keeps butterflies and moths. Since they symbolizes his desire to change into a woman.
The Kung Fu Hustle hero gets wrapped in bandages similar to a cocoon, after he discards it, his Chi finally got unblocked and his destiny awaiting. While this was happening, the audience was treated briefly to a butterfly emerging from its cocoon imargy to drive the point home.
The Butterfly Effect, the hero tries to change the present by fixing the past.
Alice in Wonderland, the character of the caterpillar is seen as some sort of symbolic function. An example, at the end of the 2010 film, the titular character has transformed by her Wonderland experiences somewhat, and this is symbolized by the caterpillar (Named Absolem in this adaption), appearing on her shoulders as a butterfly and doing a Fly At the Camera Ending.
The butterfly motif appears often in the I Am Legend. In the actual film, it does not amount to anything, but in the original ending, the butterfly represented that the Dayseekers are not the usual mindless, evil zombies, but rather sentient beings that had transformed into "better" forms than being an old-fashioned human. Though "better" is a YMMV.
In the 2015 live-action Cinderella, the gown from Cinderella's Fairy Godmother has butterfly decorations and motifs, symbolizing the titular character's transformation.
In 2006's The Illusionist, butterflies are used for this and all the other "Butterfly of..." tropes simultaneously.

Literature

In Freckles, by Gene Stratton Porter, Freckles observes in wonder of how a Luna moth came out of the cocoon and feels frustrated at his lack of knowledge of the creature, though he knows it's not a butterfly. This made him to ponder, until it dawns on him that he could find his answers about things.
In Making Money, one of Terry Pratchett's Discworld series, Cosmo Lavish uses a caterpillar's metamorphosis as an example of his own transformation. Pucci, his sister, informs him of what happens to a caterpillar, it dies and the butterfly grows out of its remains.
The Animorphs book The Departure heavily shows the theme of a caterpillar, which will later become a butterfly. It's when Cassie encounters a surprisingly sympathetic yeerk, one willing to give up her host body if only Cassie is willing to spend the rest of her life as a Yeerk...Or as a caterpillar, which was the closest thing at hand. This and Cassie's realization that the world isn't as black and white as she thought, signifies change.This also brings another fact that natural metamorphosis can somehow undo the Mode Lock of remaining more than two Earth hours, this is something Cassie willingly does to free herself of her host.
In The Girl Who Chased The Moon, by Sarah Addison Allen, when Emily's wallpaper changes for the first time, it changes to butterflies, which appeared to be trapped and want to escape. She had just learned things about the town that makes it hard for her to stay. Neither her nor the butterflies manages to flee.
Mass quantities of butterflies bookend the 1960s-vintage science fiction novel The Butterfly Kid, in both cases heralding dramatic changes that are about to occur -- just to Greenwich Village at the start of the book, and then the entire world at the end.

Live-Action TV

Delenn, from Babylon 5, uses a butterfly like cocoon to turn into a half-human, half-minbari. While transforming, the security team exploited the trope by telling the treacherous Delenn, that though she became beautiful as she emerges from the cocoon, as though she was a butterfly with wings.
In an early episode of Lost, a moth coming out of its cocoon was a metaphor for Charlie weaning himself out of drugs. Locke made this very explicit.
In ABC's What Would You Do? episode titled "Bridezilla," the bride asks her bridesmaids to wear violet dresses, were she has appended butterfly wings, a symbol of her transformation via marriage.
In Doctor Who, the Second Doctor compared his regeneration as a renewal, similar to a butterfly emerging from its chrysalis. That's why the ring no longer fits.

Doctor:I'd like to see a butterfly fit into its chrysalis case after it's spread its wings!
Polly: Then you did change!
Doctor: Life depends on change and renewal I've been renewed. It's part of the TARDIS. Without it, I couldn't survive.
Polly to Ben: It's a very different Doctor, then.

Kamen Rider Fourze does this in The Movie with Inazuman, who's another Shotaro Ishinomori creation. First, Saburo Kazeta is a bitter man, who hates "normal humans" due to them shunning him because of his psychic powers, which he includes transforming into a larval Sanagiman. Fourze, too, gives him a heart to heart (that explicitly includes a butterfly metaphor). This mad Saburo pulling a Heel Face Turn, then at the point Sanagiman "molts" into Inazuman, even having radiant wings to drive the point.

Music

The Marilyn Manson album Antichrist Superstar is a story about a child changing into the Anti-Christ, a controversial rock star. The song cycle are broken into three parts all according to an insect's life cycle. Even the album's art depicts him in stylized versions of the three stages, including with a set of wings.
Ego Likeness using a dragonfly instead of the usual butterfly for its album Dragonfly, which has a theme about personal rebirth and the cover depicts the eponymous insect.
The Metamorphosis album, Hillary Duff used this as a metaphor for its final page.
With her first album, Mariah Carey uses a similar metaphor after she divorced from Tommy Mottola, her manager. The album's name is Butterfly.
Rie Fu's song "Butterfly" is about personal transformation. The song is in one of the two albums which she released to celebrate her ten years of being a musician. It's official music video shows clips of women holding up signs with advice that they wish they knew when they were young.

Newspaper Comics

In one of the Mutts strips, Mooch just saw a caterpillar go through a metamorphosis, it inspired some thoughts about change, like that someday Earl might become a cat.

Other

Groups relating to Transgender issues often have butterflies in their logos, and some of the Transgender pride flags have a butterfly on them.

Tabletop Games

Dungeons & Dragons 1st Edition
- Player's Handbook. The Polymorph Other Spell, which can changed a creature into another type. Its material ingredient was a caterpillar cocoon.
- Oriental Adventures supplement. The Force Shapechange spell causes shapechangers to shift into their alternate form. The spell's material ingredient was a caterpillar cocoon.

Theater

In the stage musical of Charlie and the Chocolate Factory, Willy Wonka's I Am What I Am song, "Simply Second Nature," where he explains his inborn need to make beautiful things - Though weird to some. One point, while waving his cane suddenly reveals a butterfly perched on it, this continues in this adaptation's running motif of humble things that contain/conceal great beauty and possibility (This ties in to the Central Theme of imagination and its' transforming powers).

Video Games

In the Persona series, Philemon uses the golden butterfly as his symbol; he's a god that allows the heroes to tap into their Personas. In later games, according to Word of God, he appears as a blue butterfly, this implies a similar, yet lesser function. Though neither of the characters or any narrator identifies him.
In Mega Man X2, part of Morph Moth's gimmick is this trope. He begins the fight in a cocoon suspended on a thread, after taking in enough damage he, metamorphoses into a fully grown robot moth.
In Pale Blue, the Cocoon organization uses butterflies as symbolism of their goal, which is to transform humanity from its supposedly ugly "larvae" state into beautiful butterflies through their actions. To drive the point even further, their leader is even named after the Monarch butterfly.
BioShock 2, Dr. Lamb's extremely altruistic views are symbolized by a blue butterfly, this motif is worn by most of the Rapture family members, and it appears in most of Rapture as well. Even the graffiti have the butterfly motif and a written message: "WE will be reborn in the cold womb of the ocean. She is our savior" Turns out to be insanely literal than you think, since Lamb's plan of using her own daughter as a "database" of Rapture's population to create a human completely driven to the group's benefit entirely.
In Gears of War's fourth entry, JD Fenix and Kat were talking about how a cocooned butterfly changes just before they go on a raid. This foreshadows the Locust Horde's reveal that they were not totally destroyed at the third game's end. But have returned in stronger forms and converting humans into the Swarm through cocoons.
In Fatal Frame II, All God's Village has butterflies everywhere. Since the motif is associated with the twins and referring to the death and rebirth interpretation: The souls of the sacrificed twins literally becomes butterflies. When Mayu is released from her "cocoon" of her body by Mio's red, wing-shaped handprints which are left on her throat.

Visual Novel

In Culpa Innata, a revolutionary group uses butterfly symbolism heavily.

Web Comics

Sinfest: After seeing a butterfly, to Fuschia it means that people CAN change (Which she'll have to do to win her love). Her aspirationsare reveal thatshe's fascinated by them.
xkcd's Transformers parody by having them pupate.
Darths and Droids: the Separatist general, General Grievous gives an explanation of why he became a cyborg by saying "The caterpillar sees not its destiny, except by fulfilling it. Only by shedding our previous bodies can we become beautiful butterflies."

Western Animation

During the end of Bambi II, Bambi has lost his spots, antlers are starting to grow, and more importantly, his relationship with his father has improved. In some nearby entwined sprouts, a butterfly lands on it.
In Angel's Friends, Dolce uses her familiar, named Butterfly, to transforms into a human and back.
Miraculous Ladybug has a literal example: Hawk Moth utilizes this to feed off of people's negative emotions in order to turn them into Akuma.