Grendel is the first antagonist we meet in Beowulf, but he's not the last, nor the mightiest. In fact, he's just the first of a cycle of three tests.
Grendel's mother, out to avenge the death of her son, is the second antagonist Beowulf must face, but she's not the mightiest. In fact, she's not as strong physically as Grendel, but she's more dangerous for Beowulf because he must follow her into her lair in a cave beneath a lake.
Beowulf's final antagonist is completely inhuman: a fifty-foot dragon, angered by a thief and marauding around Geatland.