Cupcakes

Cupcakes is a fan-written ~4000-word short story featuring a homicidally insane Pinkie Pie. It has been noted for its dark and violent imagery. It is neither intended nor suitable for young audiences.

Plot summary
Rainbow Dash is invited to the bakery by Pinkie Pie to help her out with some baking. Once she arrives, she discovers no sign of any work in progress, but, knowing Pinkie Pie, doesn't feel surprised and readily eats a cupcake offered by Pinkie, thinking of it as of a "taste test". In reality, the cupcake appears to gave been injected with a soporific, which knocks Rainbow Dash unconscious. After the drug's effect is over, Rainbow Dash awakens to find herself tied up in a dark cellar room. Pinkie Pie appears to tell Rainbow Dash that she's going to make cupcakes, but she's out of a necessary ingredient: Rainbow Dash herself.

The rest of the story describes a series of medical procedures, almost all of them sans anesthesia, undertaken by Pinkie Pie to put Rainbow Dash to death. In the end, Pinkie Pie decides not to make cupcakes from Rainbow Dash, but instead to make a stuffed Rainbow Dash from her friend's body. The original ending also depicts Apple Bloom as taking part in "making cupcakes" as Pinkie Pie's apprentice and/or assistant.

Alternate endings
There are several alternate endings, written by other people, claiming that:
 * The whole story was Pinkie Pie's nightmare.
 * It all happened because of Princess Celestia being cursed during her confrontation with Nightmare Moon (the story of Celestia fighting Luna is described to be much more violent than remembered in pony myths of Nightmare Moon, featuring global pony-to-pony face-off, massive battles and a dragon, still being held in Celestia's palace in Canterlot, waiting for her to pass the ruler's right to Luna and letting him put an end to her life - and the curse - in the last honorable battle).
 * Twilight Sparkle accidentaly goes into the bakery, notices something weird and witnesses Pinkie Pie's brutal death in the torture table gears.
 * Celestia's Everfree Forest Rangerhoof SWAT storms the cellar, prevents Pinkie Pie from applying lethal damage to Rainbow Dash, and takes her into custody.
 * Rainbow Dash escapes by burning Pinkie alive.

Origins and impact
The original story is credited to one "Sergeant Sprinkles: World's Greatest Party Clown". It was originally posted on the /co/ board on 4chan, from where it quickly spread to other pony sites. "Cupcakes" has gained enough notoriety to garner it its own lengthy subpages on KnowYourMeme and TVTropes.