The Mountain Troll's shadow looming over the destroyed girls' lavatory
Mountain Trolls are a species of troll living, as the name suggests, in mountainous areas. In Harry Potter and the Philosopher's Stone, Professor Quirrell lets in a Mountain Troll on Hallowe'en as a diversion. Harry and Ron subsequently save Hermione from this troll in the girls' lavatory.
One of the Underground Chambers also holds a Mountain Troll, but it has already been knocked out by the time Harry, Ron and Hermione reach it.
From the Story[]
"Mountain trolls are massive, unintelligent creatures. They tower over humans (Harry and his friends meet one that is twelve feet tall), with overly long arms and short legs that are thick as tree trunks. They have dull, grey skin, lumpy bodies, and small heads, and give off an odour like old laundry and unclean toilets."