I think the monster in Beowolf was a bigfoot.
Really? Interesting. (Are you kidding or for real? - it is cool either way)
Or a surviving Neanderthal.
Beowulf was set in Denmark and Southwestern Sweden. Grendel was not identified, but I’s say he was a troll, or started out as a troll. The poem was transmitted in West Saxon. I don’t know if there were trolls in the Saxon England bestiary. Not certain if the Norse troll swam and lived in caves under water, but apparently they could in the Anglo Saxon imagination.