Symbolism, Imagery, Allegory
If you blink, you'll miss the references to the Zodiac in each chapter of Grendel. Luckily for us, there are clever scholars who have noted the appearance of these symbols and how they help to structure the narrative.
In the first chapter, Grendel "locks horns" with a ram. It's spring, and the creature just wants to hook up with a mate and preserve the species. Grendel hates him for being a brute, an unthinking beast. Okay, so you've probably got it: the ram represents the sign Aries. There are other signs to look out for, too; some of the more obvious ones are Taurus (the bull), Virgo (the virgin), Capricorn (the goat), and Pisces (the fish). The dragon falls under Leo (the lion).
To Everything, Turn, Turn, Turn...
Sure, it's cool to catch these kinds of references, but what does it mean for us as we read the novel? For one thing, it's clear that the movement of time is a huge concern for Grendel. He hates the boredom and stupidity of the predictable seasons. There's nothing that annoys him more than how the inhabitants of the natural world (including himself) are slaves to the movements of the year: it's just another sign that no creature on earth really has free will or the ability to break away from "brute" impulses.
There's also the feeling that the seasons will keep spinning past, regardless of what happens to Grendel. It doesn't matter if he sinks further into wicked deeds or if he becomes a saint. The cosmos doesn't really care if he wins his war against Hrothgar—or if he dies and disappears. It'll just keep rolling on.
That's almost more than any monster can take.