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