The three monstrous creatures that Beowulf fights are Grendel, Grendel's mother, and the Dragon. All three of them represent an inversion of Anglo-Saxonic core virtues.
1. Grendel is described as Cain's descendant, which means he is the devil's lot, fatherless (as he is not worthy to bear a name of a father), and bearing the sin of fratricide (killing one's own brother). He kills and devours 30 warriors in their sleep, which is a coward's act. He is also bloodthirsty - he doesn't kill because of honor, but because he is a carnivorous beast.
2. Grendel's mother is a female villain. We don't get to know many details about her physical appearance, which makes her more of an archetype than an actual character. She wants to revenge her son, which means she acts according to the unwritten rules of blood feuds. But in Anglo-Saxon culture, it was not a woman's duty to act on revenge. It is a male prerogative.
3. Dragon, the last villain Beowulf fights, represents some kind of inversion of kingly virtues. At the moment, Beowulf has been ruling for over 50 years. Of course, he is a just and righteous king, generous toward his warriors and subjects. On the other side, the Dragon is a destructive, irrational force that poses an ultimate threat to the kingdom. He jealously guards the treasure and attacks the realm because a single cup has been stolen from him. He is the exact opposite of what Beowulf represents, and that makes him Beowulf's antipode.
