Grendel is one of the monsters that the poem's hero, Beowulf, must fight against. He is portrayed as a man-eating, demon-possessed monster of sorts. He loves violence for the sake of violence and attacks King Hrothgar's mead-hall every night.
The first time I read "Beowulf" I pictured Grendel as a legitimate monster. Big claws. Pointy ears. Tail. Etc. I no longer think this, though. The reason for that is because the poem states that Grendel is a descendant of Cain.
"Grendel, who haunted the moors, the wild Marshes, and made his home in a hell Not hell but earth. He was spawned in that slime, Conceived by a pair of those monsters born Of Cain, murderous creatures banished By God, punished forever for the crime Of Abel's death."
The reference is Biblical. Cain is guilty of fratricide, which is killing your own brother. Cain killed Abel, and was banished by God. His descendants would then bear the mark of Cain. The Bible doesn't explain what this mark is, but it clearly made Cain identifiable to anybody who saw him.
Because Grendel is a descendant of Cain or a member of Cain's tribe, I believe that Grendel is human. So Grendel is a demon possessed, man eating, violence loving monster of sorts. Man eating? Cannibalism. He sounds a lot like a modern day serial killer psychopath. Come to think of it, he sounds a lot like Hannibal Lecter. Beowulf vows to kill Grendel, and in a wrestling match rips off Grendel's arms. Grendel runs away but is later found dead after having bled to death.