The worship of a Horse Goddess under the name of Epona appears to have come from Gaul, her worship spreading throughout Europe around the time of the Roman Empire. There were other Goddesses who were associated strongly with horses, such as the Celtic Rhiannon and so the assimilation of myth and archetype was quite straitforward. The Romans, particularly the cavalry legions, also adopted Epona whom they honoured under the name of Epona Augusta or Epona Regina. Interestingly enough the Romans usually adopted the local deities and gave them names of Gods and Goddesses they knew already, for instance the local deity Sulis (who gave her name to the city of Bath, "Aquae Sulis" in Latin) being honoured by the Romans under the name of Minerva. Epona was the only Celtic deity to be honoured in Rome, her feast day being held in December.
Her shrines were, predictably, often found in stables. Lucius Apuleius in his book "The Golden Ass" a late Roman work, mentions such a shrine. The hero, turned into a donkey in a magical experiment that went badly wrong, notices some roses, which he needs to eat to restore his human form; these roses are wreathing:
"...a little shrine of the Mare-headed Mother, the Goddess Epona, standing in a niche of the post that supported the main beam of the stable."5
From this it seems that Epona was more of a folk-deity, a Goddess of the common people who was honoured in her Mother-Goddess aspect rather than in a more warlike aspect as she is reported to have been by the cavalry legions of the Roman Empire. It would appear that some Egyptian influence was creeping into the worship of Epona as the Godess is described as being "mare headed". However as "The Golden Ass" has it's foundations in the mysteries of Isis then this is really only to be expected.
Equally other aspects of myth and legend seem to have spread across Europe with her, in an echo of the right of ascension of the Irish king witnessed by Giraldus Cambrensis a Greek writer, Agesialios, writes of Epona's creation in these terms:
"A certain man, Phoulouios Stellos, who hated women, had relations with a mare. In time she gave birth to a beautiful girl whom she named Epona."6
Notice that it's the mare that names the child, presumably she is divine herself (as the Greeks were quite familiar with their Gods taking on animal forms to become on intimate terms with certain mortals) or at the very least magical. I doubt very much that the writer of that passage plucked it out of thin air and I would suggest that he probably heard about, or maybe even witnessed a similar ritual of birth, mating and death as Giraldus was to some nine hundred years later.
As to the specifics of how Epona was worshipped records are, as always, difficult to come by. In England it has been suggested that some of the white horse figures are connected with her worship. This is certainly possible, particularly with figures such as the White Horse of Uffington, which also have bird-like features (the Uffington Horse appears to have a beak-like head) and birds do feature strongly in connection with Epona, particularly in her Celtic aspect where she was worshipped as Rhiannon, and in mainland Europe where cranes were also sacred birds of The Goddess. However arguments still rage over the White Horses but certainly the area around Uffington, the Vale of White Horse 7 and the North Downs have very strong associations with horses - Waylands Smithy is not too far away along the Ridgeway and the area is saturated in legend and folklore. I can personally attest that Epona's power is still very present in this corner of England's green and pleasant land.
As I mentioned earlier Epona, or a Goddess with firm equine connections, was worshipped under a variety of names in various places. In Celtic Wales she is known as Rhiannon and in her we see her association with the dead, as Rhiannon was Queen of Annwn, the land of the dead (which wasn't a place like the Christian Hell, rather another country where the dead went to after departing this world). She is often spoken of as guiding the dead and her birds (as I have mentioned birds do seem to feature prominently with Horse Goddesses) were said to "Sing the dead to life and the living to sleep". In the great Irish Legend of CuChullain, Loegaire, his charioteer, says of The Otherworld:
There is also a tale from the Mabinogion, which tells of how Rhiannon, wrongly accused of murdering her son, is forced to sit by the horse-mounting block by the castle gate, tell the story of her supposed murder to passers by and offer to carry them into the castle on her back - in other words to be their horse. She was saved from this fate by Tiernyon who owned a beautiful mare which had a foal every Beltaine8 - notice the date? This foal always disappeared until Tiernyon stayed up on the third time his mare was due to foal only to see the foal being grabbed by a monstrous clawed arm which he hacked off. On rescuing his new-born colt he found a child beside it which, surprise surprise, turned out to be Rhiannon's son, who later became known as Pryderi (which in Welsh means "trouble"). It is thought9 that this legend relates to a time when Rhiannon's Horse Cult was poorly treated or persecuted as later legend has her married to the sea-deity Manawyddan ap Lyr rather than to the mortal Phwyll.
Today echoes of Epona's worship still remain. In Padstow on Beltaine the "'Obby 'Oss", a wooden headed black horse figure with a man inside it, dances though the town accompanied by a retinue of musicians and revellers, the 'Oss chasing the young and not so young ladies of the village. Local legend has it that if the 'Oss catches them that it bodes well for their marriage prospects or foretells that they will soon have a baby; again this echoes the fertility aspects that appear to underlie all horse-worship. Interestingly an echo of death and rebirth that we have seen in other Horse Deity rituals is enacted in the Padstow dance when the "Day Song" song becomes the dirge and the 'Oss "dies", only to come back to life when the song changes again. The hobby horse figure can be found in many folk rituals throughout Britain and Northern Europe such as the Hodening (hooded) Horse of Kent and eastern England, the Lair Bhan of Ireland and Mari Llwyd of Wales (which mean White and Grey mare respectively) and the Schimmel or white horse of Germany. In all of these festivals, these stubborn survivals of the rituals of the Old Gods, the aspects of fertility and plenty are there for all to see. This was apparent to the early Christian Church who, of course, sought to ban them. St Augustine in the 5th century spoke out, condemning the "filthy practice of dressing up like a horse or stag" and later in Scotland the ecclesiastical authorities forbade "any man from dressing as a horse or wild beast and dancing widdershins in the Kalends of January, for this is devilish."
Fortunately they failed in their attempts and, in one form or another, Epona is still being honoured today.