Not much is written on the goddess Epona. All that researchers and archaeologist has learned about her is from the 343 inscriptions that have been found through-out the Celtic lands and especially in Rome.
She was worshiped through-out Rome and especially by the Roman Calvary who adopted her into their beliefs. The Roman Calvary consisted of men from Gaul who were great horsemen and some think were already paying homage to the goddess. They brought the goddess to Rome with them.
Archaeologist has found many stables in the Roman empire with shrines dedicated to Epona. The shrines were simple niches cut into the wall with a small statue of the goddess with garlands of roses around her. Epona also had a shrine in the barracks of the Imperial Bodyguard. German legions made plaques depicting her in human form with a foal or feeding foals.
She was considered the goddess or protector of horses, donkeys, mules, animals, riders and stables. A lot of the statues and figurines of Epona shows her riding on a horse, with foals, dogs or birds following her. She was also known as the Mistress of animals. Some believe she had a role with the Underworld because she was shown holding keys which may be the keys to the Underworld. The birds that accompanied her were often thought of as symbols of a happy other world.
In some traditions she was thought of as a fertility goddess because with some figurines she is shown with a horn of plenty, the cornucopia or a basket filled with fruits, corn and apples. Some think that she once was a fertility goddess but through time she was 'reduced' to a horse goddess.
The Romans held an annual festival in her honor on Dec. 18th as a day of feasting. She was worshiped as Epona Augusta or Epona Regina, which was invoking her on the behalf of the emperor at the time.
Other symbols and signs of Epona was the horse whip and harnesses.
During the time that the Romans were in Britain, they brought with them the goddess Epona but the goddesses Rhiannon and Macha was already a part of a horse cult before Epona appeared.