It has been suggested that he derives from the Gaulish god Ogmios and is cognate to the Irish hero Oghma Grianainech.
[1] Eufydd appears in a number of Welsh texts, spelled variously as Euuyd, Eueyed, Euyd and Ieunydd.
He appears twice in the Book of Taliesin; first in Prif Gyfarch Taliesin in which it is stated: and then again in Marwnat Aeddon: The implication is that Eufydd was remembered in Welsh tradition as a skilled magician, intimately associated with his more illustrious brothers Gwydion and Gofannon, and with his uncle Math fab Mathonwy.
Eufydd also appears in the genealogical tract Bonedd yr Arwyr (The Descent of the Saints) in which he is listed among the children of Dôn.
It has been suggested that Eufydd can be identified with Hefeydd Hen, the father of Rhiannon who appears prominently in the first branch of the Mabinogi.