There are three established incarnations of Grendel in literature.

Beowulf -- The original Grendel was a monster in the Anglo-Saxon epic Beowulf. Descendant of Cain, mere-dweller, and with the meanest mother around, he was the hero Beowulf's first foe in the work.

Gardner's Grendel -- In 1972 (?) John Gardner published the novel Grendel, a retelling of Beowulf from Grendel's point of view. He portrayed the "ridiculous hairy beast torn apart by poetry" sympathetically, and walked him through the Great Concepts of western civilization, considering them from the outside with an existentialist edge.

Wagner's Grendel -- In the back of issues of his comic Mage, Matt Wagner began a storyline about criminal mastermind, assassin, anti-hero Hunter Rose, a.k.a Grendel. After a single short book and several separate short story arcs about Hunter Rose, the series spun out into a 50 (?) issue epic spanning centuries and the world.