We don't hear much in the way of a physical description, and this is
because it's what Grendel represents that is the horror for the original
audience. His name is associated etymologically with "ground" or
"bottom," but more importantly, consider the following:
We hear some material after the battle with Grendel that introduces the
female perspective in roundabout ways, including Hrothgar's insistence
that if Beowulf has a living mother, imagine how proud she'd be.
We also hear, in a tangential story (the Finnsburg lai), about a grieving
woman whose offspring have been murdered. So Grendel's mother's
perspective is alluded to very obliquely. She snatches not skads of
drunken warriors, but Hrothgar's favorite; so however arbitary that seems
to have been in terms of her attack, she is functioning essentially in a
blood-feud. Beowulf becomes the invader into her hall just as Grendel was
in Heorot. Women in Anglo-Saxon culture are "peace-weavers" (because one
can convince oneself that arranged marriages will prompt feuds to simmer
down) and "cup-bearers" (because they fetch more booze for the men).
That's it. Grendel's mother is the opposite of what that culture values
We keep hearing a term of 50 years mentioned: especially in reference to
Beowulf's kingship and to the time the dragon has guarded its hoard.
Beowulf is now the king -- the "ring-giver" who ideally distributes booty
captured in battle to his thanes in accordance with their deserts. Good
kings are ring-givers and bad kings (again we hear tangentially of a
couple) are miserly. The dragon functions then as the opposite of a good
king because it guards the trreasure but can do nothing with it. It
represents malice, destruction, and greed -- the dark side of kingship.
So it's another döppleganger situation in effect. Both the dragon
and Beowulf die in this final battle, and the last images are those of
waste and desolation.
When Beowulf is defending his reputation from Unferth's accusations in front of Hrothgar and the rest, he mentions having to fend off "niceras" -- sea-monsters. My Chaucerian and Medievalist college professor, Thomas J. Garbaty said in 1984 about these things: "Nicoras. What are they? I don't know. They're sea-monsters; they're bad. You gotta kill them."