His name is apparently Welsh or Cymric for "Red Giant." There's also an article on him in The Oxford Dictionary of Celtic Mythology, by James MacKillop>> Welsh giant who boasted of the number of kings he had killed by wearing their beards. A place-name story explains that Arthur killed him and commanded his men to place stones over the body, forming Gwyddfa Rhita [Rhita's Cairn], an archaic Welsh term for Snowdon.
The giant who collected the beards of kings, and who challenged King Arthur and was defeated by him, appears in Geoffrey of Monmouth's History of the Kings of Britain. There he is called Retho, and lived on Mount Arvaius (Mount Snowdon). He was the strongest opponent Arthur had ever encountered until many years later he fought against the Spanish giant at Mont St.Michel, France (in a failed attempt to rescue a damsel in distress - she died when the giant tried to rape her).