Rhiannon was a Welsh goddess of sovereignty and of horses, principal female figure of the First and Third Branches of the Mabinogi. She first appeared to Pwyll, prince of Dyfed, who had been sitting on the magical mound of Arberth where strange things were said to happen: a beautiful woman in golden silk rode past on a white horse at a slow walking pace, but no horse of Pwyll's company could catch her, however fast they galloped, until at length Pwyll himself called out to her and she stopped, scolded him for not having spoken sooner, and revealed that she was Rhiannon, daughter of Hyfaidd Hen, betrothed against her will to a man she did not want and seeking a husband who would help her escape him. Pwyll married her, after a not entirely creditable sequence of events involving Rhiannon's tricking the rival suitor at the wedding-feast.

Their son Pryderi was stolen on the night of his birth by a mysterious force; the nurses, accused, smeared the sleeping Rhiannon's face with the blood of puppies they had killed and accused her of having devoured her own child. Rhiannon was sentenced to a curious penance: she was to sit at the mounting-block outside the gate of Arberth, tell her story to every visitor, and offer to carry them on her back into the court. The penance was endured for many years before Pryderi was eventually recovered (he had been raised by the family of Teyrnon Twrf Liant, who had recovered him from the magical claw that had stolen him).

The image of Rhiannon as horse-woman, of the goddess associated with horses, and of the curious penance involving riding-on-the-back has connected her in the modern scholarly literature with the Continental Celtic horse-goddess Epona (treated next). The two are linguistically and functionally similar enough that an underlying Indo-European horse-goddess of sovereignty (the king's marriage to whom secured his right to rule) has been reconstructed; whether the connection is direct genetic descent or typological parallel is debated.