Rhiannon: Her Conception As"Horse Goddess" and "Sea Goddess"

As I already commented on a previous post, it is acknowledged that the Mabinogion is the most valuable written source where the character of Rhiannon appears.

Rhiannon features in both the first and third branches of the Mabinogi, the tales of:

• Pwyll Pendefig Dyfed

(a tale that revolves around his friendship with the ruler of the nether-realm, his finding and gaining Rhiannon as a wife, the birth, loss and re-discovery of his son) &

• Manawyddan fab Llŷr

(a tale that recounts how Pryderi ,-who survived the war in Ireland-, offers his own realm to Manawyddan and gifts him his mother, Rhiannon to be Manawyddan's wife). 

"Rhiannon: The Horse Goddess" 

Let’s recall that when Rhiannon gave birth to her son, Pryderi, at the Winter Solstice, he disappears while in the care of six of Rhiannon's ladies-in-waiting , and the servants panicked. They schemed and decided to put puppy blood on Rhiannon’s face as she was sleeping, and scattered puppy bones all around her.

Rhiannon is forced to do penance for her supposed crime, since everyone believed Rhiannon had eaten her baby.

As punishment, she humbly stood at the castle gates with a horse’s collar around her neck.

So the story of how Pryderi vanished mysteriously and was then reborn from the womb of a horse illustrates that belief.  Although the myth of Rhiannon appears to be a straightforward story, in which a Goddess is falsely accused of murdering her child and suffers unjust penance imposed on her by humankind, it is the history of the cult of Rhiannon.  In which she is venerated as a great queen and mother; her cult is corrupted and she is degraded to her most basic form, the horse; and then her cult is revived as her son is reborn, but she changes into the Goddess of the underworld through her association with the sea god's son Manawydan fab Llyr.

The Celts did absorb some of the older faith into their own, taking basic deities and complicating them with myths.  There was a tradition whereby the King had to reaffirm his place by first mating with a sacred white mare and then cooking and eating her flesh and bathing in the water that was left after the horse's bones were boiled. 

The mare was the symbol of the Goddess and the King had to become Her consort in order to rule.  The Mother GOddess was the conduit through which the souls of the dead could pass into the Summerlands and be reborn again.

"Rhiannon: The Sea Goddess"

The only relation specificially mentioned between Pwyll and Manawydan is that Pryderi gave his mother Rhiannon in marriage to Manawydan after thedeath of Pwyll, Pryderi's father and Rhiannon's first husband. Manannan mac Lir'Manannan, Son of Lir', was an Irish sea-god (OI leir, ler, 'sea, ocean')and eponymous hero-god of the Manxmen, whose island was named after him. He also is directly related to the Welsh sea-god Manawydan fab Llyr. Like many of the primal IE ocean gods, Manannan was older than the Danann sky gods, yet was considered one of them.

Though Manawyddan is involved in almost all the events of the Mabinogi of Branwen ferch Ll±r he is there as an observer. His only real role is to be one of the seven survivors of the Brythonic invasion of Ireland to return to Britain with Brân's head. His story really starts in the next tale, the Mabinogi of Manawyddan fab Llŷr.

After the events of the Mabinogi of Branwen ferch Llŷr, Manawyddan is the only man left without a realm and Pryderi (who also survived the war in Ireland) offers his own realm to Manawyddan and gifts him his mother, Rhiannon to be Manawyddan's wife.

Related Sources:

http://ancientworlds.net/ 

"The Religion of the Ancient Celts" by J. A. MacCulloch (1911)

"The encyclopedia of Celtic mythology and folklore"  by Patricia Monaghan (2004)
http://branawen.blogspot.com/2011/12/rhiannons-celebrations-eponas-day-mari.html