Grendel, Grendel's mother, and the dragon all appear in the epic Anglo-Saxon poem Beowulf.
The Geats and the Danes depicted in the epic were Christian, shown through the Christian perspective the epic was told from. That being said, there are many references made to the fact that the Geats and Danes followed God and honored him through their epic battles and buildings which they erected (Heorot).
Therefore, any of the battles depicted in the text took place between a hero of God (Beowulf) and a monstrous, God-hating foe (Grendel, his mother, and the dragon). Evidence of this hatred is found in the following lines:
Thus the clan's life was one of good cheer and revel until that fiend of hell began to work evils. Grendel was this grim beast called, who haunted the moors and secluded fens; this accursed one had long dwelled with monsters since the Creator had decreed his exile. On the kin of Cain did the sovereign God avenge the slaughter of Abel; Cain gained nothing from this feud and was driven far from the sight of men for that slaughter. From him awoke all those dire breeds: ogres, elves, and phantoms that warred with God a lengthy while; He paid their wage to them!
Therefore, all of the monsters depicted in the text symbolize the pagan creatures and images which needed to be fought and destroyed in the name of God.