Grendel and his Mother

Grendel is an unusual monstrosity that appears in Norse mythology. He is depicted as a huge, hairy, violent humanoid that savages and eats men. In the epic poem Beowulf, the monster repeatedly attacks the title hero's allies, killing and consuming many, until he is challenged by Beowulf himself, who rips of its arm. Grendel dies of his wounds, but his mother, a "sea-hag," tries to avenge him by attacking the same hall frequented by her son. Beowulf hunts her down as well and kills her with a sword at the bottom of the lake where she lives. It is strongly hinted that both Grendel and his mother are aquatic or at least amphibious, as they both live in a cave at the bottom of a lake.