Cave-troll

When the Fellowship of the Ring explored the Chamber of Mazarbul in Moria, Gandalf mentioned that he had seen one or more cave-trolls in the passage outside. The huge arm and shoulder with a dark skin of greenish scales and the great flat toeless foot that appeared in the gap in the barred door of the chamber presumably belonged to this cave-troll. After Frodo Baggins had stabbed the foot with Sting, black blood dripped from the blade to the floor and smoked there. Later during the flight of the fellowship through Moria, two trolls appeared in the Second Hall of Moria, but it is not recorded if they were cave-trolls.

Mentioned only once in the written records of Arda, nothing else is known of these creatures. One can only assume that this breed of troll lived in some of the caves of Middle-earth. The fact that Gandalf was able to recognise the creature he encountered in Moria specifically as a cave-troll strongly suggests that this appearance was shared by others of their kind.