@Platus
When the Nightmare Spirits who had possessed Luna were exorcised by the Elements of Harmony, they fled back to the Moon and adapted it for their needs. In the process, the cute little animals who already lived there were transformed into monsters. "Miss Buffy" is one of them. Only her physical appearance was changed by the Nightmare Spirits; she's still gentle and harmless, and Fluttershy has befriended her.

@AC4
In the original, it's implied that Twilight, Applejack and Rainbow are all swearing at each other.