Madawg ap Uther, Madoc, Maduc
This name appears on several occasions in the Arthurian tales. Welsh legend makes him a legendary Prince of Gwynedd, who was supposed to have discovered the Americas and been the ancestor of a group of light-skinned, Welsh-speaking Indians in the American west.
Arthur’s brother and the father of Eliwlod. He also makes an appearance as the son of Uther in the Book of Taliesin, though this reference would seem to be simply a translator’s error. In the Welsh poem Ymddiddan Arthur a’r Eryr, Eliwlod, Arthur’s nephew, appears in the guise of an eagle.
This character’s father is named as Madoc, which would imply that he was King Arthur’s brother-in-law.
Source
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century