Two entries with the name Constantine.
Constantine
Custenhin, Custennin
Son of Mynwyedig and father of Goreu in Culhwch and Olwen. Constantine was the shepherd of Ysbadadden, the giant father of Olwen, and he gave lodging to Culhwch and his party during their visit to Ysbadadden’s realm.
He was apparently married to one of Culhwch’s aunts. Ysbadadden had slain twenty-three of his sons, so he kept the last, Goreu, hidden in a chest.
The Triads call him Arthur’s cousin. This identification plus his residence in Cornwall suggests that he is a Welsh prototype of Constantine.
Sources
Culhwch and Olwen | Late 11th century
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century
Breudwyt Rhonabwy | 13th century
Constantine
Custenhin
Father of Erbin and grandfather of Geraint.
Source
Geraint and Enid | 13th century