Symbolism, Imagery, Allegory
Grendel has a hard time finding his place in the world, and this is made even more difficult by watching the brute, unthinking creatures who seemed to live and breathe without one iota of angst. The book actually opens with Grendel screaming at the sky, "Why can't these creatures discover a little dignity?" (6).
Well, they can't. The ram needs to copulate; the bull sees Grendel stuck in a tree in his pasture and has to charge; the goat has to climb—it doesn't matter that he's climbing on Grendel's rocks or that he's just had his brains smashed to bits. On the other hand, the humans are this weird mixture of brute animal and intelligent planner. The humans make it difficult for Grendel to understand the world and his purpose in it.
Why? Because Grendel is neither simply brute beast nor brainy strategizer. Though it doesn't seem like it, this monster has a pretty strong moral code: he thinks about the consequences of his and others' actions and judges accordingly. He wants to be part of the human community, but he can't seem to squash his knee-jerk anger at humans.
And let's face it: eating babies and old ladies isn't going to get you into the human club.
The animals of the natural world symbolize the part of Grendel's nature that he wishes to deny and that he can never fully embrace. He isn't far enough down the food chain to be absolved of his crimes, but he isn't far enough up it to make any changes to his role as monster.
That's how he sees it, at least. We're not so sure that Grendel's necessarily in the wrong about everything. It's the humans, after all, who insist that he's a monster—and we know they're at least partially wrong. Maybe the humans don't want to admit they have anything in common with a monster, just as Grendel doesn't want to admit he has anything in common with the animals.