What is the name of the monster Beowulf fights in Heorot?

The monster Beowulf fights in Heorot is Grendel. Heorot is the great mead hall built by Hrothgar, king of the Danes, and Grendel attacks it repeatedly, terrorizing the hall’s warriors and making it unusable.

In the poem, Grendel is disturbed by the sounds of celebration and singing coming from Heorot. He attacks at night, killing and devouring warriors. His assaults continue for twelve years before Beowulf travels from Geatland to help Hrothgar. Beowulf fights Grendel without weapons and tears off the creature’s arm, mortally wounding him.

Grendel is one of Beowulf’s three principal antagonists, alongside Grendel’s mother and the dragon. These figures are easy to confuse because Beowulf fights each in a separate major confrontation. Grendel’s mother attacks later to avenge her son, while the dragon appears near the end of Beowulf’s life. The “sea monster” is not the answer to this question: Beowulf does have an underwater battle, but that fight is against Grendel’s mother.