What is "cynddylan"

Cynddylan, or Cynddylan ap Cyndrwyn was a seventh-century Prince of Powys associated with Pengwern. The son of King Cyndrwyn, Cynddylan is described in the poem Marwnad Cynddylan (Elegy of Cynddylan) as a king of Dogfeiling, a sub-kingdom of Gwynedd near to Rhuthun in the middle of Denbighshire, in modern-day Wales.