AU

AU, or Alternate Universe is a popular genre and concept used in fanfiction to denote when a story takes place outside of the typically accepted canon of a work.

In fanon
In My Little Pony fanfiction, AU is most often used to tell a story in an alternate timeline or "what-ifs". Common what-ifs tend to involve major events, such as what-if was hatched by a different pony, what-if Princess Luna defeated Celestia and actually brought about the Eternal Night, what-if Scootaloo lived a terrible, miserable life for some reason (this one is super popular), etc. The amount of potential "divergence point" in the show are countless, with many possibilities.

In canon
The show, moreso the comics, have played around with the idea of "what-ifs" on numerous occasions. The one-shot comic My Little Pony: Deviations dealt with the what-if that Prince Blueblood was Celestia's pupil rather than Twilight Sparkle and thus, lead into how the Mane Six (minus Twilight) would defeat Nightmare Moon without her aid.

In addition, the Reflections arc and first chapter of My Little Pony: The Manga additionally deal with several what-ifs.