Goreu
Goreu fab Custennin is not a shapeshifting spirit but a hero of the Welsh tale Culhwch ac Olwen in the Mabinogion: son of Custennin the giant and cousin to Arthur, he joins Culhwch's quest and, at the story's end, beheads the giant chieftain Ysbaddaden, whose fortress and lands he receives. His name means 'best'.