Which Welsh god is known as a trickster and magician?

The Welsh god known as a trickster and magician is Gwydion.

Gwydion fab Dôn is one of the central figures of the Fourth Branch of the Mabinogi, a medieval Welsh mythological cycle. He is described as a magician, hero, and trickster, and much of his story concerns his nephew Lleu Llaw Gyffes, whom Gwydion helps after a series of magical and political disasters.

His trickery is not merely comic. In one episode, Gwydion disguises himself and his companions as bards to obtain pigs from Pryderi, king of Dyfed. The deception leads to war, and Gwydion kills Pryderi in single combat. Later, he uses magic to help restore Lleu after Lleu is transformed and grievously wounded.

Gwydion is sometimes loosely called a Welsh god, but surviving medieval texts present him primarily as a powerful mythic magician and member of the family of Dôn. He should not be confused with Math fab Mathonwy, another major magician and king in the same branch.