A breed of troll, Mountain Trolls are known as biggest and heaviest one in their species. Those creatures which usually seen in grey can weight up to one tonne. Mountain Trolls usually prefers walking but some species of them could use Graphorns as a vehicle of transport.

Mountain Trolls could understand human language but because of their small brains, they are usually a dunce and they have tendency of violence and destruction. All of the troll breeds carries a big wooden club with them.

Where it Mentioned?

• Harry Potter and Sorcerer’s Stone (First Time – Book and Movie) A Mountain Troll which enter the Hogwarts causes big panic. It neutralized by Potter and friends in girl’s bathroom.
• Harry Potter and Cursed Child (Book)

Sources

• General information about the creature (Pottermore)