""Horsegoddess" or "Divine Mare".
Gallo-Celtic Horsegoddess.
Epona is the only gallo-Celtic goddess, that made her way into the Roman empire-pantheon, where she was highly worshipped - especially as the protectress of horses and foals. Almost every stable had a shrine for her and she was very famous in the Roman cavalry. In earlier times she must have been an incarnation of fertility as the Divine White Mare.
Epona seems to have almost no relations in the Celtic pantheon. I didn't find any story about husband, children or so. This could be a sign that she was originally a great nature-mother goddess of her own right and not only 'reduced' to be a horsegoddess. Without having a husband or children she lives in the tradition of a virgin selfproducing great mother.
She is sometimes addressed in the plural form Eponabus, which seems to be a strong indication that she stands in relation with the Three Matres/Matrones and was herself maybe thought to be three-formis and a goddess of fertility.
Epona is the protectress of horses, animals, riders and stables. She is the Mistress of animals. Her many fertility attributes make her a Mothergoddess, which was maybe only reduced later to the horse-aspect. Or: The great importance of horses for the celts and the symbol of the fertile mare giving birth to a foal and taking gentle care of it as a loving mother led the celts to worship in her nature and fertility in general.
Eating horse-meat on normal occasions was a strong TABU.
Maybe it was common to swear in her name (source: satire of Juvenals). One prayed to her for help in misery. Her cult had no special mystery and was popular.
Her annual festival in Roman times was around the 18. December (in Mantua/Italy).
Sacred animals: Horses (particular: mares and foals), birds and dogs.
Whip, harness and the Key to the doors of the Otherworld.
Basket filled with fruits, corn and especially apples,
Horn of plenty.
Cornucopia ("horn of plenty") which suggests that she could (originally) have been a fertility goddess..
Rosegarlands were put around her pictures and shrines.
According to many pre-christian roman sources Epona had a shrine in almost every stable of the Roman empire.Still in the christian middleage she was worshipped as a kind of holy maid for the horses.
One of the main centers and maybe the origin of her cult seems to have been East-France with the city Alesia.
Only in Roman times she was brought to the Britain Islands and worshipped there, but Rhiannon and Macha show that the horsecult was famous on the Britain Islands before that.
Even in Rumania and Yugoslavia there were found a lot of inscriptions worshipping her. Spain also has some statues and inscriptions."
http://inanna.virtualave.net/epona.html