Cunedagius
According to Geoffrey of Monmouth, king of Britain in the eighth century BC.
With his cousin Margan, he rebelled against Queen Cordelia, his aunt, imprisoned her, and took her throne. Cunedag and Margan divided the island between them, but Margan eventually attacked Cunedag and was slain.
Cunedag became king of all Britain and ruled for 33 years. He was succeeded by his son Rivallo.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138