John Koch, in The Historical Encyclopaedia of Celtic Culture, says that the name Rhiannon “indisputably derives from Old Celtic *Rigantona (divine queen)… an exact counterpart of Teyrnon <*Tegernonos, ‘divine king”. Koch also refers to the name ‘Modron’ deriving from Matrona (‘Divine Mother’) and suggests links with the Gaulish horse goddess Epona and another Gaulish goddess Rosmerta. He speculates whether such correspondences suggest various goddesses with overlapping attributes or one goddess with several names or epithets which become names.
W.J. Gruffydd in his study of the First and Third branches of Y Mabinogi : Rhiannon (Cardiff,1953) discusses the development of the Rhiannon story from earlier myths.
Using linguistic evidence to trace the name Rhiannon back to its Brythonic form *Rigantona, he equates this goddess with Matrona mother of the god Maponos. Brythonic Matrona and Maponos would develop into Modron and Mabon in Welsh and these are the names found in the medieval Welsh tales.
Gruffydd concludes that “bearing in mind the modern form of the name Rigantona, Rhiannon is Modron on horseback.” (p.104) Gruffydd provides a long summary of his suggested process of change from the original myth to the medieval story (pp 109-112) which follows here in abbreviated form:
(a) The myth of the Great Queen Rigantona whose cult is associated with that of Epona the Horse Goddess. She was portrayed sometimes in the form of a horse, sometimes surrounded by foals, and at other times with the trappings of a horse, such as a horse collar. In less barbaric portrayals she was represented as a woman riding a walking or trotting horse. Her consort was the ‘Great King’, *Tigernonos.
(b) The myth of the Great Mother, Matrona, whose son Maponos was stolen from her by the King of the Other-World. While Maponos was in the Other-World, a great darkness and desolation fell upon the land, and his mother wandered over the earth seeking him. When she found him, light and life were restored to the world.
The legends of Rigantona and Matrona were fused together in the early stages, so that in Wales Rhiannon and Modron had, at least in one respect, an identical history. It thus came about that Rhiannon’s son, like Mabon, was snatched away into the Other-World and a great desolation fell upon the land until he was rescued by Rhiannon’s consort Teyrnon (<*Tigernonos). The rescue has been preserved in Pwyll.
Rhiannon retains her association with horses and asses. It is probable that her offspring had been represented as a foal, or the foal associated with her in statuary was understood to be her son. This foal is lost, and rescued by Teyrnon. At the same time, in the Modron form of the legend, the son is a human child.
Stages Four to Seven
These mainly consist of folklore elements grafted on to the developing tale in the manner of oral story-telling processes as the story took on its own life separate from the original myth.
(a) The Rhiannon story is specifically located in Dyfed (S.W. Wales) and becomes part of the legendary history of Pryderi, a hero associated with this area.
(b) The Mabon-Modron story (not yet localised in Dyfed) adopts Rhiannon’s son as the lost child.
Stages Nine to Twelve
Other additional material and further confusion of the roles of some characters further complicate the story.
Finally all these elements come together and are inherited by the author of Y Mabinogi who made his own attempts to iron out inconsistencies and explain contradictions in telling the story.
This sort of attempt to reconstruct mythological origins is now out of favour with many scholars. But the origin of the name Rhiannon in the constructed name *Rigantona is based on good philological evidence.