Hmmm... a possible lead on the identity of Gwythyr ap Greidawl?
(Source: Wikipedia.org)
A man by the name of August Hunt made some claims, found HERE and HERE, over an obscure Romano-British God Veteres, identifying this figure with the character of Gwythyr ap Greidawl, rival of Gwyn ap Nudd and father of the second Gwenhwyfar, wife of Arthur.
Fo what it's worth, the locations where this deity is reported are all found in the areas near the Hadrian Wall in Northern Britain (Northumberland, Durham and Tyne-and-Wear).
In Culhwch and Olwen, Arthur is said to have travelled North to resolve the dispute between Gwyn and Gwythyr over Creiddylad:










