Philosopher's Stone]], when [[../../Characters/Professor_Quirrell|Professor Quirrell]] let one into the dungeons during the Hallowe'en Feast. A second one, also provided by Quirrell, was one of the obstacles guarding the approach to the Stone
In [[../../Books/Prisoner of Azkaban/Chapter 14|Harry Potter and the Prisoner of Azkaban]], when [[../../Characters/The Fat Lady|the Fat Lady]] returns to watch the Gryffindor common room entrance, a squad of security trolls is recruited to keep her safe. These are apparently not much smarter than the one that broke up the feast two years earlier, but are apparently more biddable.