For twelve years, before encountering Beowulf, the monster Grendel had been embroiled in a war with humans. Before that, as a young monster, Grendel lived in a cave in a marsh with his mother who gave up language long ago. Their cave is at the edge of civilization but Grendel swims a lake of firesnakes and, on the other side, encounters the human world. Being a nearly immortal monster, Grendel watches the historic parade of mankind as it grows and changes and Hrothgar of the Danes becomes king.
The words in the original manuscript that describe Grendel and his mother are variations on roots used to mean wolf. According to linguists, wolf has the meaning of an animal and a second meaning on an outlaw who has committed a crime of such a nature that it puts the individual beyond forgiveness or redemption. A second root has the meaning of demon. Both these together paint the picture of Grendel as an outlaw with an animal nature who is a nearly immortal demon.
For more information on the linguistics pertaining to Grendel, read Grendel on monstropedia.org