In the Demiplane of Blood: Part of the Tier 1 Demiplane event "Redfang"; hits for a max ~1,500; backstabs for ~3,000; flurries. They are slowable and mezzable, and can be killed with root spells. They spawn in bunches of five on top of three random people. Four seconds after being rooted, an emote goes off and the rooted bat despawns.
In the Tower of Frozen Shadow: Part of the final encounter within the Tower of Frozen Shadow anniversary raid (see this quest entry for raid information). These can be killed in the same manner as in the Demiplane of Blood.