In Welsh mythology, Gilfaethwy, son of the goddess Dôn and brother to Gwydion and Arianrhod, features in the The Fourth Branch of the Mabinogi: Math Son of Mathonwy.

His uncle, Math fab Mathonwy, ruler of Gwynedd, must always have his feet in the lap of a young virgin, Goewin, except during wartime. Gilfaethwy desires Goewin and conspires with Gwydion fab Dôn to steal pigs from Pryderi, king of Dyfed, inciting conflict between their realms. While Math is away at war, Gilfaethwy rapes Goewin, enraging Math, who punishes them by transforming them into various paired animals, illustrating the savagery of their deeds. Gilfaethwy spends a year as a hind deer, mating with Gwydion as a stag. Subsequently, they become a boar and sow, then wolves, each year producing offspring for Math. After three years, Math releases them from their punishment and restores their humanity.

Gilfaethwy plays a minor role in Welsh legend, likely serving to enhance the narrative of his more prominent brother, Gwydion.

In Arthurian romance, Gilfaethwy’s character inspires figures such as Sir Griflet, appearing as Girflet fils Do in Chrétien de Troyes’ Erec et Enide. Scholars note the transformation of Dôn from a goddess into characters like Do, portrayed as the castellan of Carlisle and chief forester in Arthurian tales.

The Lore mentioned in connection with Don refers to Florée, the Flower Bride, an Arthurian counterpart to Welsh Blodeuwedd and Irish Blathnat.