We know very little about what, exactly, Grendel looked like.  We know he had arms and legs (or Beowulf could not have "arm-wrestled" him and wrested his arm from his shoulder socket), and that the hand was claw-like.  We know he carried a pouch he had apparently made (one version simply says "skins," while another translation says "dragon skins") and into which he placed his victims.  We also know he was huge, as it took four grown men to carry just his head (staggering as they walked), and that his body could not be penetrated by swords (though that was more about the spell he cast against all weapons than about his actual skin).  That's about it.

There might be several reasons we don't get anything more specific; these are the two most likely to me:

1.  Effective description is difficult when you're trying to be impressive.  For example, if I told you something was really scary and then I go on to describe it to you, the odds are good you would not find it as frightening as I did.  What scares or appalls one does not necessarily do so to another.  Poe understood this and, in "The Pit and the Pendulum," he never describes or explains what's in the pit so no one can say, for example, "Snakes?  They aren't scary."  Or "Rats?  Who's afraid of rats?!"  These kinds of reactions take the focus off the point--the pit is a horrifying prospect...so horrifying, in fact, that being pressed against a wall of  red-hot steel is the better option.  We have enough detail about Grendel to understand the threat he would be if he came to our homes, but not so much we get stuck on the believability of every detail.

2.  The story isn't about the monsters, it's about the battle between good and evil.  We know enough to be clear that Grendel is evil, and that's all we need.