Cryptozoologists have speculated on whether some form of human-like primate, be it a surviving Neanderthal or an unknown species, has been behind European folklore about "wildmen" and "trolls." Grendel of Beowulf fame pops up often in this conversation. The reason is that some translations, like a prose version I read in high school, explicitly make Grendel a wildman of some sort. The version I remember (and cryptid researcher Dale Drinnon agrees we read the same book) makes him about seven feet tall, "drooling with spit, stinking and hairy." Does that accord with the actual text of the poem?
We have, of course, no definitive version of the poem, whose origins are lost in time. Indeed, since for a long time it was a tale told or sung orally, and changed constantly, there is not really an official nor an original form. But the most authoritative source we have, the one all modern translations spring from (at first, second, or third hand) is a single surviving manuscript in the British Library. When poet Seamus Heaney went back to this source and used it directly for his wonderful new translation, he shed some light on Grendel.
That Grendel was a seven-foot hairy wildman appears to be a later author's interpretation. Heaney's translation of Beowulf doesn't contain anything sufficiently descriptive of Grendel to be of much use. Heaney, in an introduction, provides his view after reading the sole surviving original text: that Grendel makes us think of "some hard-boned, immensely strong android frame, half Caliban and half-hoplite." (Caliban being the deformed (also not well described, but usually depicted as looking like a bestial subhuman or wildman) yet eloquent slave in The Tempest, while a hoplite was a Greek heavy infantryman.)
Grendel is "a fiend out of hell, a grim demon" from "the banished monsters, Cain's creatures." He attacks Beowulf with his claws ready, which doesn't sound primate-like, and the later description of the arm and hand Beowulf tears off doesn't sound mammalian at all. On the other hand, Grendel is sufficiently human to have a soul, which is condemned to Hell.
The description of his size is not consistent: he is bested by one human hero in unarmed combat, but a few pages earlier, it says he bore off THIRTY men at a time, which would make him gigantic. Nowhere is he compared directly in size to a man or anything else that would allow us to ascribe an approximate height to him. All we know for sure is that he could fit through the doors of the mead hall.
From a cryptozoological perspective, I don't think there is anything solid we can make of this. Grendel is a little-described fiend who, given the reference to the old idea of Cain's clan as outcasts from humanity, might plausibly be assumed to be hairy, even if the poem makes no further reference to it. His mother seems to be something else again: she's been seen with him stalking on the moors, but she moves better in water than on land.
They went a different direction with the recent film, which follows the poem about halfway before veering off into a different story. In this visually stunning but ultimately less than compelling version, Grendel is a huge, misshapen parody of a human, but is still a physical animal, while his mother is a supernatural entity who can be physical (in more ways than one) when she wants to be.
I've always thought that Grendel was useful only as evidence of humanity's long fascination with semi-humanoid monsters, and not as evidence of any specific animal that was then living, or indeed had ever lived outside the human imagination. This doesn't mean such things can't exist, only that we can't rely on an old Anglo-Saxon poem to prove it.
But what a poem it is.