Custenin Gorneu, that is Constantine of Cornwall, is a figure in Welsh genealogies, a descendant of Cynan the reputed lord of the earliest British settlers in Little Britain and the father of Erbin, father of Gereint. The Latin name Constantinus normally changes to Custennin in Welsh.
In some later versions of these genealogies, e.g. Mostyn 117, he is made to be also the father of Uthyr Penndragon (Uther Pendragon), the father of Arthur, thus identifying him with the Geoffrey’s King Constantine of Britain.
Mostyn 117 is written in the same hand as the earliest fragment of the tale Gereint fab Erbin.
From Bonedd y Saint:
Kybi, Mab Selyf, m. Gereint, m. Erbin, m. Custennin Gorneu. Cybi, son of Selyf, son of Gereint, son of Erbin, son of Constantine of Cornwall.
... Pedur m Cado m Gereint m Erbin. Gereint m Erbin m [Kustenhin m] Kynỽaỽr m tudwaỽl m Gỽrwaỽr m Gadeon m Cynan m Eudaf hen. ... Peter, son of Cado, son of Gereint, son of Erbin. Gereint, son of Erbin, son of [Constantine, son of] Cynfawr son of Tudwawl, son of Gwrwawr, son of Gadeon, son of Cynan son of Eudaf the Old.
From Mostyn 117, 5 [the name Eudaf should follow Kynan]:
Arthur m. Vthyr m. Kustenhin m. Kynuawr m. Tutwal m. Moruawr m. Eudaf m. Kadwr m. Kynan m. Karadawc m. Bran m. Llyr lletieith. Arthur, son of Uther, son of Constantine, son of Cynfawr, son of Tudwal, son of Morfawr, son of Eudaf, son of Cadwr, son of Kynan, son of Caradawg, son of Brân, son of Llŷr Half-speech.
In Gereint son of ErbinEdit
If Constantine is the father of both Erbin and Uthyr Benndragon, then Erbin will be uncle to Arthur son of Uthyr Benndragon. And so it is related in the Mabinogion, in the story of Gereint son of Erbin (as translated by Gwyn Jones and Thomas Jones):
‘God prosper you,’ said Arthur, ‘and God’s welcome to you. And whence do you come?’ ‘We come, lord,’ said they, ‘from Cornwall, and we are messengers from Erbin, son of Custennin, thy uncle. And our message is to thee, and greetings to thee from him, even as an uncle should greet his nephew and as a vassal should greet his lord, ...
Some modern scholars take the Custennin Gorneu of the genealogies to be identical to the Constantine whom Gildas upbraids and whom Geoffrey of Monmouth makes to be King of Britain following Arthur.