I think that some of the Roman cavalry had shrines to Epona. I think her worship was more widespread than Rhiannon's.
I find faint traces of a Father Horse and Mother Horse cult coming to Britain--Horst and Hengsta (sp?). The British (and USA) still have deep seated taboos against eating horseflesh which are not shared in nearby countries (the French, Belgiums, etc. will eat horse.) I think that in the British Isles eating a horse was reserved for high holy rituals, specifically for transfering kingship. The Roman Catholic church did all it could to wipe these rituals out in its push against paganism, but the taboos against common people eating horse still seem to operate.
I could never find other pagan dieties who were specifically dedicated to horses, though one in each group would tend to deal with horses along with his or her other duties, like Neptune the sea god.
You might want to get Linda Kohanov's "Way of the Horse" book and card set. She mentions, among others, Horse Headed Demeter, Poseidon (greek sea god) as father of Pegasus, Chiron the centuar, and others, as well as Epona. I still see this set at Borders in the horse section.
Morgan Llywelyn wrote the book "The Horse Goddess", a novel about the human Epona, who served as a basis for the goddess. Interesting book, and Llywelyn is a good writer. Probably not what you are looking for though.
It is very hard to find much out about pagan deities except for the Greek and Roman ones. Most of pagan Europe was illiterate, and the pagan Celts who were literate considered it sacrilege to write down anything about holy matters. We are left with the surviving myths and tales from the oral traditions. Very frustrating.
Rhiannon's tale is in the Welsh "Mabigonian" (sp?), just one of many tales within this work.
Thanks for this info, Jackie. I have one of Linda's books somewhere. Good reading. I also have her on the dvd "The Way of the Horse" put together by Stormy May. I made the mistake of loaning this to someone and have not gotten in back yet. Have you seen it?
Poseidon fathered Pegasus on the Medusa (snake haired minor goddess at the time of the Greeks.) The tale is more complicated than that, I think I remember Medusa's head being cut off by a Greek hero.
Sorry I'm not more specific, I have this horrible cold.
I looked up Pegasus in Robert Graves "The Greek Myths". Medea was a beautiful Gorgon before Poseidon mated with her. But the mating took place in a temple to Athena, and, enraged, Athena turned beautiful Medea into a horrifying monster whose look could turn men into stone.
Pegasus also has a brother born at the same time (when the Greek hero Perseus cut off Medea's head), a human warrior named Chrysoar, who later had a son. Just think, there may be humans running around descended from Pegasus's human brother!
Medusa came from Libya, in north Africa.