Grendel

Origin
Grendel was a powerful troll-like monster that terrorized the great hall of Heorot, provoked by the singing of the men who lived there. He came in the night, while everyone was sleeping, and ate the warriors of Heorot. The warriors, in turn, were helpless against the strength and impenetrable hide of Grendel. Eventually, Grendel drew the attention of the Geat hero Beowulf, who came to hunt down the monster. Beowulf and Grendel engaged in a battle that shook the great hall, but eventually Beowulf managed to rip Grendel's arm off and the monster fled to the marshes from which he came and died.

Later his mother came to Heorot to avenge her son. While she was even more powerful than Grendel himself, she too was hunted and killed by Beowulf. Both Grendel and his mother apparently lived in a cave at the bottom of a lake, where the remains of their victims could be found.