With a name deriving from Rig Antona or 'Great High Queen', Rhiannon is the goddess of horses, who is known as Epona in Gaul and as Édáin Echraidhe and Macha in Ireland. Her totem animals were the bull and three cranes, animals that have associations with death and rebirth.

The daughter of Hefeydd Hên, Rhiannon was extremely beautiful, a beauty that led Pwyll to become enchanted by her. He sought her hand, a union that was agreed to by both Rhiannon and her father. However, at a feast to celebrate their betrothal, her rejected suitor, Gwawl fab Clud, entered in disguise and claimed her as a boon from Pwyll, which he unwittingly granted.

Rhiannon was furious but was compelled to comply. However, she suggested that the proposed date for the fulfilment of the boon be put off for one year and then concocted a plan with Pwyll. The plan led to Gwawl fab Clud being captured in a sack and kicked viciously by Pwyll's men until he begged for mercy. Pwyll and Rhiannon released Gwawl fab Clud only after he had renounced his claim to Rhiannon and had promised never to seek revenge.

Rhiannon and Pwyll were married, but, as she had not produced a child after three years, Pwyll's advisers beseeched him to take another wife. He refused and within a year Rhiannon had borne a son. As she slep that night, her ladies-in-waiting also fell asleep and the child, who had yet to be named, mysteriously disappeared. Fearing for their own safety, the ladies-in-waiting smeared the sleeping Rhiannon with blood and woke Pwyll, saying that his wife had killed their newborn son. Pwyll refused to accept that his wife could possibly have done such a thing, but, as she would not defend herself, was forced to make her do penance, by carrying all those who accepted her offer into their castle.

The child had meantime been discovered on the doorstep of Teyrnon Twryf Liant, who took him in, called him Gwri and treated him as his own son. After a year, struck by the child's resemblance to Pwyll, Teyrnon Twryf Liant took the infant to the King, who was overjoyed to be reunited with his son, who was called Pryderi. Pwyll died a short time later. Pryderi grew into a handsome and brave warrior under the guidance of his foster father Pendaran Dyfed. He succeeded his father and was much loved by his people, eventually marrying Cigfa, the daughter of Gwyn Gohoyw.

Pryderi accompanied Bendigeid Vran on his expedition to Ireland against King Matholwch, and was one of the seven survivors who returned to Britain carrying the severed head of Bendigeid Vran. In his absence, however, his cousin Manawydan fab Llyr, the rightful heir to Bendigeid Vran, had been dispossessed by Caswallawn, the son of Beli. Pryderi therefore gave Manawydan fab Llyr his mother Rhiannon as his wife along with the kingdom of Dyfed.

Rhiannon, together with her son and their hunting dogs, was later trapped in a caer under an enchantment cast by Llwyd fab Cil Coed to avenge Pwyll's ill treatment of his friend Gwawl fab Clud, a spell that was later broken after Manawydan fab Llyr and Cigfa had endured much hardship and had finally captured the wife of Llwyd fab Cil Coed in the guise of a mouse. As the spell was broken, Rhiannon, Pryderi and their dogs were restored to their former state, as were the lands of Dyfed.