Rhiannon is a major figure in Welsh mythology, appearing in the First Branch of the Mabinogi, and again in the Third Branch. Ronald Hutton called her "one of the great female personalities in World literature", adding that "there is in fact, nobody quite like her in previous human literature". In the Mabinogi, Rhiannon is a strong-minded Otherworld woman, who chooses Pwyll, prince of Dyfed (west Wales), as her consort, in preference to another man to whom she has already been betrothed. She is intelligent, politically strategic, beautiful, and famed for her wealth and generosity. With Pwyll she has a son, the hero Pryderi, who later inherits the lordship of Dyfed. She endures tragedy when her newborn child is abducted, and she is accused of infanticide. As a widow she marries Manawydan of the British royal family, and has further adventures involving enchantments.
Like some other figures of British/Welsh literary tradition, Rhiannon may be a reflection of an earlier Celtic deity. Her name appears to derive from the reconstructed Brittonic form *Rīgantonā, a derivative of *rīgan- "queen". In the First Branch of the Mabinogi, Rhiannon is strongly associated with horses, and so is her son Pryderi. She is often considered to be related to the Gaulish horse goddess Epona. She and her son are often depicted as mare and foal. Like Epona, she sometimes sits on her horse in a calm, stoic way. This connection with Epona is generally accepted among scholars of the Mabinogi and Celtic studies, but Ronald Hutton, a historian of paganism, is skeptical.
Rhiannon's story
Y Mabinogi: First Branch
Rhiannon first appears at Gorsedd Arberth, an ancestral mound near one of the chief courts of Dyfed. Pwyll, the prince of Dyfed, has accepted the challenge of the mound's magical tradition to show a marvel or deal out blows. Rhiannon appears to him and his court as the promised marvel. She is a beautiful woman arrayed in gold silk brocade, riding a shining white horse. Pwyll sends his best horsemen after her two days running, but she always remains ahead of them, though her horse never does more than amble. On the third day he finally follows her himself and does no better, until he finally appeals to her to stop for him.
Rhiannon characteristically rebukes him for not considering this course before, then explains she has sought him out to marry him, in preference to her current betrothed, Gwawl ap Clud. Pwyll gladly agrees, but at their wedding feast at her father's court, an unknown man requests Pwyll grant a request; which he does without asking what it is. The man is Gwawl, and he requests Rhiannon.
Rhiannon rebukes Pwyll a second time for his rash promise, but provides the means and the plan to salvage the situation. She holds a second wedding feast for Gwawl, where she deploys Pwyll's men outside in the orchard. She instructs Pwyll to enter the hall dressed as a beggar and humbly request Gwawl fill a certain 'small bag' with food. But she has enchanted the 'small bag' so it cannot ever be filled by normal means. Gwawl is persuaded to step in it to control its magic, which means Pwyll can trap him in it. Pwyll's men rush in and surround the hall, then beat and kick Gwawl as the Badger-in-the-Bag game. To save his life Gwawl is forced to relinquish Rhiannon completely, and also his revenge. Rhiannon marries Pwyll, then journeys to Dyfed as its queen.
After a happy two years, Pwyll comes under pressure from his nobles, to provide an heir. He refuses to set Rhiannon aside as barren, and in the third year their son is born. However, on the night of his birth, the newborn disappears while in the care of Rhiannon's six sleepy maids. Terrified of being put to death, the women kill a puppy and smear its blood on Rhiannon's sleeping face. In the morning they accuse her of infanticide and cannibalism. Rhiannon takes counsel with her own advisers, and offers to undergo a penance. Pwyll is again urged to set her aside, but refuses, and sets her penance instead. She must sit every day by the gate of the castle at the horse block, to tell her story to travelers. She must also offer to carry them on her back as a beast of burden, though few accept this. However, as the end of the story shows, Pwyll maintains her state as his queen, as she still sits at his side in the hall at feasting time.