"You know there’s some interesting stuff in here... there are mountain trolls riding graphorns through Hungary, there are giants with winged tattoos on their backs walking through the Greek Seas, and the werewolves have gone entirely underground—"
-- Hermione Granger suggesting that Harry should read the reports on his desk (CC1).
Trolls are large, ponderous, human-like magical creatures who exist on the fringes of wizarding society. There are three types of trolls: mountain, forest, and river (FB).
The mountain troll is the largest and least intelligent of the various types. It stands twelve feet tall, with grey skin, a lumpy body, and flat horny feet. It exudes a powerfully awful smell, “a mixture of old socks and the kind of public toilet no one seems to clean.” Its nose is full of what looks like lumpy, gray glue: troll boogers (PS10).
Trolls have been known to ride Graphorns (FB, CC1).
Troll language supposedly consists of grunting and pointing (GF7), but that description is courtesy of the Weasley twins, who were attempting to wind up Percy on the subject of his boss' linguistic ability at the time.
Quirrell, to create a diversion so he could go after the Philosopher's Stone, let a mountain troll into the castle on Hallowe'en, 1991. It wandered around the corridors until Harry and Ron locked it in a girls' bathroom. Right after they did that, they realized that Hermione Granger was in that particular bathroom, and they had no choice but to charge in after it. With a lucky combination of derring-do, desperate magic, and sheer dumb luck, they managed to knock it out and save Hermione (PS10).
Quirrell, it turns out, has something of a knack for handling trolls. He placed one in the chambers guarding the Stone, knowing he'd be able to knock it out quite easily when the time came. By the time Harry, Ron, and Hermione arrived in that chamber, the troll was out cold (PS16).
Tom Riddle alleged that Hagrid, as a boy, would go into the Forbidden Forest to wrestle trolls (CS17).
Trolls participate in wizarding society to some extent; some witches and wizards make a career out of training security trolls (OP29).Dumbledore hired security trolls to guard the Fat Lady 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 (PA14). Umbridge apparently also hired security trolls to guard the Firebolt she had confiscated from Harry (OP30).
Trolls are not recognized as magical beings (FB). Pierre Bonaccord, the first Supreme Mugwump of the International Confederation of Wizards, wanted to stop troll-hunting and give them rights, but his appointment to that office was contested by Liechtenstein, since they were having problems with a tribe of very dangerous mountain trolls at the time (OP31). The Black family kept an umbrella stand made out of a troll's leg in their front hall (OP4). A large stuffed troll can be found in the Room of Requirement when it is in "lost objects" mode (HBP24).
Hagrid and Madame Maxime ran into a couple of mad trolls on the Polish border while on their journey to visit the giants (OP20).
The Norwegian Quidditch team is accompanied by a troupe of performing trolls (Pm).
Note that OP31 does not capitalize "mountain troll" in the U.K. edition, so the term should not be capitalized.
A troll is a supernatural being in Norse mythology and Scandinavian folklore. In Old Norse sources, beings described as trolls dwell in isolated rocks, mountains, or caves, live together in small family units, and are rarely helpful to human beings.
Later, in Scandinavian folklore, trolls became beings in their own right, where they live far from human habitation, are not Christianized, and are considered dangerous to human beings. Depending on the region from which accounts of trolls stem, their appearance varies greatly; trolls may be ugly and slow-witted, or look and behave exactly like human beings, with no particularly grotesque characteristic about them.
Trolls are sometimes associated with particular landmarks, which at times may be explained as formed from a troll exposed to sunlight. Trolls are depicted in a variety of media in modern popular culture.