In John Gardner's Grendel, in chapter seven, after Grendel has been exposed to the Shaper, Grendel, the narrator, writes:
Pity poor Hrothgar,
Pity poor Grendel,
That's poetry. It may be bad poetry, or doggerel, as I've heard it called, but it is still poetry. In fact, the novel records Grendel's progress as a poet, featuring the narrator's poetry several times, including his final poem, spoken while the stranger with a chest as big as an oven (Beowulf, of course) bangs Grendel's head against a wall, impressing upon the monster the actuality of the empirical world:
The wall will fall to the wind as the windy hill
will fall, and all things thought in former times:
Nothing made remains, nor man remembers.
And these towns shall be called the shining towns!
You can see the progress Grendel makes as a poet.
In fact, though, the novel is itself presented as an attempt by Grendel to be an artist. Yes, he uses rhyme, imagery, alliteration in his poetry, and the complexity of his poems grows as he progresses. He moves from poetry that reads like a nursery rhyme and is dominated by trochees to more sophisticated iamb-centered verse. But Grendel is an artist, not only a poet. His poetry is only a small part of his attempt to be an artist.
Grendel writes a portion of the novel as screenplay, for instance, and the screenplay, by the way, includes much poetry.
Grendel's effort to create art is a record of his search for meaning, his search for a way to live his life. Each chapter reveals a different philosophy, a different way to live. And his art, and since you asked about it, his poetry, is an attempt to treat or investigate life, or as it is more formerly called, "human" existence. Perhaps more than anything else, this is the quality that makes Grendel a poet: his attempt to investigate existence in his art.