Atepomarus in Celtic Gaul was a healing god. Mauvières (Indre), Apollo was associated with this god in the form Apollo Atepomarus.
At some of Apollo's healing sanctuaries (as at Sainte-Sabine, Burgundy) small figurines of horses were associated with him.
The root "epo" refers to the word for "horse", and the epithet is sometimes translated as "Great Horseman" or "possessing a great horse".
References
Atepomarus Wikipedia(Text) CC BY-SA