It’s reconstructed (hence the asterisk) Brittonic, the ancestor of modern Welsh, Cornish and Breton – and would have been very close to Gaulish. Latin and Gaulish were quite closely related in linguistic terms and some have theorised that both the Italic and Celtic languages came from a common original branch of the Indo-European family tree, which they name Italo-Celtic. If you change a Latin “kw” to a Gaulish “p” you get equus (horse) turning into “epos”. Curiously, the Italic languages seem to have a p/q division similar to the Celtic languages.
We know of Epona through inscriptions in Latin and Greek from the Roman period, however, she is definitely Gaulish/Celtic in origin and seems to have been very popular during the Gallo-Roman period. The Romans took her cult to Rome where she was venerated as a protectress of their cavalry units among other things. Interestingly, she seems to be the only non-Roman goddess that was adopted widely throughout the empire and was not ever “married off” to a Roman deity either.
Let’s not forget that the Romans, Celts, Etruscans, Greeks and myriad other peoples didn’t live in a bubble, they inevitably influenced each other and were influenced from outside too. There are common themes running through many of their mythologies.
My own feeling is that Epona is a Romanised Gaulish memory of *Rigantona, but I need to do more research on this one, so I stand to be corrected.