Security troll

Trolls participate in Wizarding Society to some extent; some witches and wizards make a career out of training security trolls. Albus Dumbledore hired security trolls to guard the Fat Lady portrait after Sirius Black attacked her. These were evidently a different type of troll than the mountain variety, since they seemed considerably more intelligent and less smelly. They spent all their time pacing the corridor in front of the Fat Lady's portrait, giving dirty looks to everyone who happened by and comparing the sizes of their clubs. Dolores Umbridge apparently also hired security trolls to guard the Firebolt she had confiscated from Harry Potter.