Personally, I have always imagined Grendle as a person, but a person who isn't limited to human limitations. In Beowulf, it reads: "Snatched up thirty men, smashed them..." This tells us that he can lift more than a human can but later the story tells us that he slips through the door. Maybe he can change size or maybe he isnt even really physically real. Either way, Grendle may look like a man, but he is far from it. I also like to think that Grendle wears clothes like the thanes does, symbolizing that he is similar to them, and that he can appear in multiple places as once, like when he is attacking the Herot. He is attacking men and stacking them up but he can also notice that the old blind man jumps out the window. If he could terrorize multiple people at once, his terrifying levels would be off the charts.