Mynyddog Mwynfawr (the Wealthy) was the celebrated leader of the Northern British armies honoured in Taliesin's poem, 'Y Goddodin'. His background is uncertain, but he was probably a brother or elder son of his predecessor as King of Din-Eitin, Clydno Eitin. Clydno's son, Cynan, was certainly one of the leading warriors amongst his retinue. Mynyddog's people were of Gododdin (Lothian), though he probably only ruled the western portion of this kingdom, being part of an usurping dynasty from Strathclyde.
In AD 598, King Mynyddog gathered a large host of warriors around him at his capital of Din-Eitin (Edinburgh). Over the previous decade, the Northern Yorkshire Dales, around Catraeth (Catterick), had been lost to the Saxon settlers from Deira, despite nominal Rhegedian overlordship since the death of the last Kings of Ebrauc (York), eighteen years before. Mynyddog was determined to strike a blow for British sovereignty and take them back. He sent his men south to push back the heathens. It is not clear whether he went himself. They numbered three hundred, while the Deiran and Bernician armies they met at Catraeth apparently reached into their thousands. The slaughter was unspeakable, yet, despite being outnumbered, the British were technically victorious. Only three of their number survived, whilst the enemy was wiped out.
Mynyddog was apparently married to a daughter of one Eudaf Hir (the Tall), by whom he had a son named Dumnagual; though he may have been succeeded by Cynan ap Clydno, one of the survivors of the great battle.