Trixie starts out the story as The Atoner, who wants to learn true magic from Twilight in order to make up for her boastful and arrogant past. When Twilight turns, she talks Trixie into helping strike at the other Elements of Harmony... then Trixie sees how powerful Twilight is, and performs a full turn herself in order to convince Twilight to keep her.
In the Pony POV Series, Fluttershy suffers a severe Break the Cutie and falls victim to the same corruption as Nightmare Moon, transforming into the Big Bad of the story arc, Princess Gaia/Nightmare Whisper.
The Nameless Passenger turns out to be a version of Twilight who turned into a vengeance crazedNightmare seeking to punish Discord for all eternity via Groundhog Day Loop, no matter how many lives she may ruin or erase in the process.
Horseshoes and Hand Grenades has Gentaro do one when Kengo didn't revive him. A being known as Ophiuchus resurrected him and brainwashed him to believe that his friends wanted him to die and left him to the hands of Ryusei Sakuta/Kamen Rider meteor. Things get very bad.