Three Unfortunate Counsels of the Island of Britain:
Meinlas is the horse of Caswallawn in Triad 38. Caswallawn is a composite of Cassibelanus, the historical leader of a rebellion against the Roman invasion of Britain, and Caswallawn ap Don, a god of the Britons.
Hengist and Horsa are the leaders of the Saxon invasion of Britian in the fifth century; Rhonwen is a Saxon princess whom Vortigern wed in order to make an aliance.
I'm not familar with the third tradition per se, regarding Arthur dividing his forces with Mordred.
Back to The Welsh Triads