Creepypasta

Creepypasta is a type of horror story that is posted and circulated on the internet, usually through dedicated websites or 4chan. In general, the concept of a creepypasta is often summed up as an "internet campfire story" or "internet ghost story", though this only really applies to older Creepypasta and a few newer ones.

History
In terms of relevance to My Little Pony, several fan-written and popular creepypastas have been written, however, most of them were not written in the typical style of a creepypasta -- as in they were presented as in-universe events, rather than the story of an individual coming across a lost or secretive episode. Because of this, they tended to be more akin to fanfiction but were still circulated in the Creepypasta sphere. Because of this, they were sometimes referred to as "Ponypastas".

The most famous of these pastas are probably Cupcakes, Rainbow Factory, Sweet Apple Massacre, Pale Luna and Li'l Miss Rarity, though countless others exist. After a certain point, the term "creepypasta" fell out of style and later similar stories were often re-labeled as "Grimdark" or simply as fanfiction. These stories tended to focus more on alternative history (in-universe) or alternate universes ("AU") that wrote of a world i which key events happened differently. The most famous of these AU-themes is probably the idea of what happened if Luna won, whether during her initial rebellion or her return, and enslaved Equestria in a nightmare regime.

As should come to no surprise, Creepypastas tended to be quite graphic and even sexual in nature, with lewd themes, but some were more child-friendly. The most famous of these are probably the flash-games, Story of the Blanks and Super Filly Adventure, the first played out like a cryptic ghost story where explores an abandoned village and comes across a dark past. Though creepy, the game has no overtly graphic themes and thankfully nothing in terms of sexual themes.

As is the case with early Friendship is Magic fanfiction, early creepypasta tended to invent new characters, organizations or other lore that was not only not present in the show at the time (or ever) but contradictory with the shows overall tone and world-building in order to justify its themes and plot. Some on the other hand tended to build upon established lore or characterization to create horror scenarios.