If Beowulf is an archetype of an epic hero, what is Grendel an archetype of?

I thoroughly agree with the answers above, but want to add that Grendel is integral to  Beowulf achieving his status as an epic hero. 

Joseph Campbell, in his classic work, The Hero With A Thousand Faces articulated the phases in which the hero must go through in order to be considered worthy of the title.  Here are a few of the key steps in which Grendel "helps" Beowulf: 

 ·  The call to adventure or to a quest for identity and the realization that the hero or heroine has special duties or responsibilities in this world.

·  Revelation of the nature of the hero or heroine's true identity and birthright and their special responsibilities.

·  The discovery of personal virtues and strengths and usually at least one great weakness.

·  The discovery and development of special powers which are unique to the hero or heroine.  These are often gifts from the gods or other powerful beings who the hero or heroine has assisted, and such gifts usually compensate for the weakness.

·  An arduous physical or psychological journey fraught with trials, testing and temptation.

·  Ultimately the hero or heroine must rely on his or her own strength, wits and resources to emerge victorious.

 Without Grendel, there would be no Beowulf. Grendel is the first and arguably toughest test for the hero.