Although the goddess Epona was originally a Celtic horse goddess, she later acquired a place in the Roman pantheon, being worshiped throughout the Roman empire (see the sculpture belong that comes from
Macedonia, for example). You can find out more at
Wikipedia.
Her name itself, which is from the Celtic language of the ancient
Gauls, means "Great Mare." You can learn more about the blending of Gaulish (Celtic) and Roman religion at
Wikipedia.
This sculpture of Epona is from
Roman Macedonia, circa 400 C.E.
This sculpture of the goddess Epona
comes from Germany, circa 200 C.E.