In Geoffrey of Monmouth’s chronicle, a son of King Morvid of Britain.
He joined his brother Iugenius in a revolt against their elder brother, King Elidur. Elidur was thrown in prison. Peredur and Iugenius divided the island, and Peredur became king of all Britain when Iugenius died. Upon Peredur’s death, Elidur was again given the throne. Peredur’s son, Runno, eventually became king.
These events occured in the third or second century BC.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138