The oldest poem in English is the Anglo-Saxon Beowulf, known from a 10th Century manuscript. Beowulf, the hero, came from Geatland in modern Sweden. His historical existence is doubtful: he perhaps lived in the late 6th Century AD. He went to the aid of King Hrothgar of Sjaelland (now in Denmark) as the latter was having his hall, Heorot, raided by the monstrous Grendel, who was eating his men. Beowulf in combat wrenched the arm off Grendel, but the hall was then raided by Grendel's mother. Beowulf went looking for her and a combat ensued in which he chopped off her head. He then found Grendel and dealt with him likewise. But what was Grendel exactly?
The poem would seem to imply that Grendel was some kind of humanoid, for it tells us he was descended from Cain (in the Bible), but this would not have been part of the original story if it had a pre-Christian origin. How did anyone know his name, anyway? One takes it he was hardly on neighborly terms with the locals.
He does seem to have been one of a race or species, for he had an equally daunting mother and the poem refers to his kin. The word Grendel may then have been a species name, perhaps related to Middle English gryndel (angry). This species may have lived on in the minds of Anglo-Saxons in England, as there are placenames such as Grendele's Beck (near Worcester), Grendel's Pit (Devon).
Grendel's Mere (Staffordshire). In these cases, however, the word may be a common noun.
If Grendel was one of a species, one can infer from the poem that he was a creature of large and burly nature and the word thurs, which means a kind of giant, was used of him. Interestingly, a thurs often had watery connections and Grendel's mother was said to live in water. Yet, for an ordinary mortal to beat him in combat, he cannot have been a towering figure.
Many parts of Europe have legends of Wild Men, creatures that somewhat resemble the famous Bigfoot. Reports of such creatures even surface in modern times. Could the Beowulf story preserve a tale of a combat with such a creature? The legendary Wildman would certainly fill the bill. Dwelling in a remote place unsettled by humans, this creature was not alone - don't forget his kin were out there. He could have been a primitive humanoid. I don't think he could have been a Neanderthal, as they did not attain a height greater than Homo sapiens. Something more primitive, say Homo heidelbergensis, could have survived long enough to be coeval with modern man. Perhaps the legend preserves memory of a combat with such a creature.
I have heard no accounts of Wildmen in modern Sjaelland, but there are still remote parts of Europe where reports of them are not unknown. Whether these reports are accurate is a matter for the cryptozoologist to investigate.