The Welsh form of Ambrosius Aurelianus, as he appears in Nennius.

An early Welsh poem notes that

before the lords of Emrys, Cei did great deeds.

A statement which is rather unclear.

Historia Brittonum | Probably Nennius, early 9th century
Pa gur yv y porthaur | Poem 31 of the Black Book of Carmarthen, probably c. 1100