Grendel is both a creature and the name of a mission during which you have to kill the creature. The mission has you seeking out Grendel's lair to find a creature who has ripped the face off of a man. The creature is a large, darkly colored Bullymong.
This is likely a reference to the Anglo-Saxon epic poem Beowulf where the main character, Beowulf, is searching for a hideous creature/man who is responsible for eating the residents of a nearby mead hall. Eventually, he finds this creature's lair and confronts Grendel.
This would certainly leave open the possibility for future easter eggs for Grendel's mom and possibly a dragon in a similar fashion as how Mothrakk from the original Borderlands lead to the Son of Mothrakk in Borderlands 2.
