Beowulf originates from a time when much of Germany and England was forested and what was not forested was often swamp or marsh land. Conan Doyle's Hound of the Baskerville's has as its non-living villain the awful Grimpen Mire from which nothing can escape. I guess (and its only a guess) that anything that can live and go and return from and to a swamp has supernatural powers. I've only ever glanced through the saga but its certainly contains a powerful imagery although I have never been able to really envisage Grendel or his mother!