We can use process of elimination to determine the name of the monster Beowulf fought.
Chaucer is the last name of the author of The Canterbury Tales. Chaunticleer is also the name of a character in that poem.
William the Conqueror is the name of a historical king that invaded England from France.
That just leaves Grendel, the name the monster that Beowulf fights in Hrothgar's kingdom.
