Good campfire story! Really
I think the origin story of the Pacific northwest/Canadian Sasquatch is better. It goes back to an isolated semi-tribe group that lived apart from their tribe as they were what could roughly be called sorcerers or witches (sp. ‘Brujos’ and ‘Brujas’).
Such sub-groups still have their own rules, and one of them broke the rules and was expelled from the group. After hard living for a time, and still practicing his forbidden crafts, he noticed that he could chase down prey without weapons. So looking in some calm water, he could see that he had become a monster.
Holding a grudge, he returned to the group and massacred all the men, and forced the women to practice his sorcery until they also became monsters - his harem.
(this version is much abbreviated from the original.)