We are used to the idea that the Celts took up Roman gods and equated them with their own. (Or that the invading Romans renamed them.) However, the process could just as easily go the other way.
The best-known instance of this is the Gaulish horse-goddess Epona, who became very popular first with the cavalry units of the Roman army, then with the Roman populace, who took her into their homes and stables. She was the only Celtic deity with a holiday in the Roman calendar: December 18th. The Romans don’t seem to have had an indigenous horse-deity (except perhaps Neptune, who had other things to attend to), but the Celts were horse-mad.
Since the goddess Rosmerta often (not always) appears with the Roman god Mercury in both inscriptions and art, it is generally assumed that she is his consort, and the images that show her with his attributes indicate that he was the more powerful partner. There is a strong case to be made, however, for reading it the other way around.