Where did the Beowulf myth come from?

The dragon was Beowulf's son with Grendel's mother.

Another Opinion

Now, that answer only applies in the movie version. In the actual myth, the dragon was just a dragon in a random tower of a long dead king. It slept until a man stole a jeweled cup from it's horde, angering it and leading it to kill. Beowulf was able to prevail against the creature, but with help from an ally named Wiglaf and eventually died from the wounds he sustained from the battle.