[cf. L montanus, mountain; W mwynfawr, rich].
Ruler of the 6th-century petty kingdom of Gododdin who raises a war-band to make war on the Angles and Saxons in Y Gododdin, with disastrous results. His capital is Din Eidyn (probably Edinburgh). A speculative translation of his name is ‘king of the mountain’; he is often known as Mynyddog the Wealthy in English texts.
From A Dictionary of Celtic Mythology in Oxford Reference.