“Can poltergeists become animagi?” McGonagall posed to her sixth years. It was an interesting question for debated because technically speaking they weren't ghosts. They were not wizards either and therefore had no magical ability.

After some discussion, McGonagall continued the lesson with: “True they cannot be animagi, but poltergeists can indeed disguise themselves and are actually talented with it.”

Hogwarts' poltergeist, Peeves, loves disguising himself in the armor to scare first years. He would hide in an armor suit; then when the students least expect it, Peeves would clang out of the armor, throwing random bits at them and just being loud and boisterous.