Ynyr Gwent was a mid-5th century monarch of the kingdom from which he took his epithet. Late tradition makes him a son of Prine Dyfnwal of Dyfed and, therefore, great great grandson of the Emperor Magnus Maximus, though this is far from certain. He was surely named after the Emperor Honorius.

In order to claim the Gwentish throne, Ynyr married King Vortimer’s heiress, St. Madrun (possibly a distant cousin), and together they founded a dynasty which ruled until the Norman conquest. Ynyr may be identified with Ynwyl of Caer-Teim (Cardiff) in the story of Gereint and Enid. The King, presumably in extreme old age, was supposedly converted to Christianity by St.Beuno, though this may have been a different man of the same name.