His uncle Math ap Mathonwy, king of Gwynedd, must keep his feet in the lap of a young virgin at all times unless he is going to war. Gilfaethwy lusts after Math's current foot holder, Goewin, so to get her alone he and Gwydion steal pigs from Pryderi, king of Dyfed, sparking a conflict between the neighboring kingdoms. While his uncle is off fighting, Gilfaethwy sneaks back to Gwynedd and has his way with Goewin. Math is furious when he discovers this, and punishes his nephews by transforming them into a series of paired animals using his great skill in magic. For a year Gilfaethwy becomes a hind deer and Gwydion a stag; they mate and produce an offspring which is delivered to Math. Next Math makes Gilfaethwy a boar and Gwydion a sow; when they return a year later with a son he makes them wolves. After the third year he relieves them of their punishment and makes them human again.
Gilfaethwy is a minor character in Welsh legend, and may have been used in the Fourth Branch simply to advance the story of his more illustrious brother Gwydion.
- Jeffrey Gantz (translator, introduction), The Mabinogion, Penguin Books, London, 1976.