| A common ancient Celtic tradition for newly-crowned kings, especially in the British Isles, involved the king being ritually married to the goddess who oversaw the region. Epona would place some kings, most often in Ireland, in the awkward position of having to "wed" a horse. Epona - her name in Gaul; she was known as Rhiannon, Macha and Etain in Wales, Ireland and England respectively - was goddess and protectress of horses.
As was typical of almost all of the Celtic goddesses, Epona had many maternal and fertility functions. Representations of her - most have been found in what is now France - show her sitting side-saddle on a horse, often with symbols of fertility such as young animals or (less often) a cornucopia. Her most important function was that of horse goddess, however. Horses were extremely valuable in ancient Celtic society, as signs of wealth, beasts of burden and war, and as a method of transportation. During the Gallic War, for instance, the Gauls occaisionally sent their cavalry from the field during sieges rather than send them into the fight to avoid losing their mounts.
In time, the Romans came to know and respect the worship of Epona. She eventually became the patron goddess of the cavalry of the Roman Empire, partly due to her beneficient, protective nature: Epona's worship imposed no restrictions on behaviour and promised protection to both rider and horse.
Sources
Green, Miranda. The Gods of the Celts. Dover: Alan Sutton Publishing 1996.
Ross, Anne. Pagan Celtic Britain. New York: Columbia University Press 1967.
Stewart, R.J. Celtic Gods, Celtic Goddesses. London: Blandford 1990. | 
