Sphere of influence: Horse-goddess, among others
Location: Gaul and the Rhineland, to begin with
Famous portraits: Many, most from the Roman times, but also the White Horse of Uffington, England
Epona is one of the most famous Celtic goddesses, and of the goddesses that were the most worshiped in the ancient world. The heart-land of her cult was eastern Gaul and the Rhineland, but it spread through the Celtic world and when the area was conquered by the Roman empire the cult spread to even to the heart of Rome and even to North Africa. In Rome she was only Gaul goddess to have a festival - on the 18th December.
She was a horse-goddess, which means she was a fertility goddess. Horses were extremely important to the Celts and fertility connected to both animals and soil. She was generally pictured with horses, sometimes with a mare and a foal - generally in the Burgundy area - sometimes between grown horses, two or more, and sometimes riding side-saddle on a mare. She is always connected to mares. She is also sometimes depicted with a basket of fruit and corn, also signs of fertility.
There are not many cult places that are known, the only positively identified shrine devoted to Epona was found in the ruins of a temple in Entrains (Nièvre, France), where two inscriptions were found.
But there was also a darker side to her, she is depicted on several tombstones, on horseback of course, where she seems to be leading the deceased to the Otherworld.
To the Romans she was a horse-goddess, not a goddess of fertility. In the Roman army she was above all popular with the Gaulish cavalary, and it is likely they saw her as a protector due to her strong connection with horses.