- "There are some things you can't share without ending up liking each other, and knocking out a twelve-foot mountain troll is one of them."
The mountain troll is a breed of troll. They are the largest and the most vicious kind of their species. They are often depicted as bald and pale-grey. They sometimes use Graphorns as mounts, which results in scars due to the Graphorns not liking it.
A band of mountain trolls were wreaking havoc in Liechtenstein, which led to Pierre Bonaccord's position as Supreme Mugwump of the International Confederation of Wizards to be challenged by the wizarding community, as he tried to ban troll-hunting and grant them rights at the time.
The Troll that Professor Quirinus Quirrell let into Hogwarts School of Witchcraft and Wizardry during Halloween in 1991 was a mountain troll. It was defeated in the girl's bathroom by Harry Potter and Ron Weasley.
The troll that was left to guard the Philosopher's Stone in Hogwarts School of Witchcraft and Wizardry during the 1991–1992 school year was also a mountain troll. It was defeated by Professor Quirrell.