Hyddwn appears in the fourth of the Four Branches of the Mabinogi

Four Branches of the Mabinogi

The Four Branches of the Mabinogi are the best known tales from the collection of medieval Welsh prose known as the Mabinogion. The word "Mabinogi" originally designated only these four tales, which are really parts or "branches" of a single work, rather than the whole collection...

 and is one of the three sons of Gwydion

Gwydion

Gwydion fab Dôn is a magician, hero and trickster of Welsh mythology, appearing most prominently in the Fourth Branch of the Mabinogi, which focuses largely on his relationship with his young nephew, Lleu Llaw Gyffes...

 and Gilfaethwy

Gilfaethwy

In Welsh mythology, Gilfaethwy was a son of the goddess Dôn and brother of Gwydion and Arianrhod in the Fourth Branch of the Mabinogi.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...

. As punishment for the rape of Goewin

Goewin

Goewin is a figure in Welsh mythology, where she has a small but crucial role in the fourth branch of the Mabinogi, named after Math fab Mathonwy...

, Math banished his nephews, turning them into a breeding pair of deer for a year, then wolves, and finally pigs. They had three children over the three years: Hyddwn, Hychddwn

Hychddwn

Hychddwn Hir appears in the fourth of the Four Branches of the Mabinogi and is one of the three sons of Gwydion and Gilfaethwy. As punishment for the rape of Goewin, Math banished his nephews, turning them into a breeding pair of deer for a year, then wolves, and finally pigs...

, and Bleiddwn

Bleiddwn

Bleiddwn appears in the fourth of the Four Branches of the Mabinogi and is one of the three sons of Gwydion and Gilfaethwy. As punishment for the rape of Goewin, Math banished his nephews, turning them into a breeding pair of deer for a year, then wolves, and finally pigs. They had three children...

. Hyddwn is the offspring of the deer

Deer

Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...

 pairing. Math took him and transformed

Shapeshifting

Shapeshifting is a common theme in mythology, folklore, and fairy tales. It is also found in epic poems, science fiction literature, fantasy literature, children's literature, Shakespearean comedy, ballet, film, television, comics, and video games...

him into a human being, baptising him Hyddwn ("Stag-man").