I went looking for the troll because I -- I thought I could deal with it on my own -- you know, because I've read all about them.
-- Hermione Granger, lying her head off (PS10)
The Mountain Troll is the largest and most violent of the various troll species. 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). Mountain trolls have been known to tame and ride Graphorns, or at least try to ride them. Graphorns don’t seem to be very keen on the idea (FB).
- Professor 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, only to realize soon after that Hermione Granger was also in that particular bathroom. With a lucky combination of derring-do, desperate magic, and "sheer dumb luck," they managed to knock it out and save Hermione (PS10).
- 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).
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"Mountain Troll" from Harry Potter Wiki