Salvete

Regarding Apuleius' mention of the shrine of a Mare-headed Mother, that may have been borrowed from Pausanius "Periegesis" to add some local flavor to Apuleius' story. Both works were written around the same time. Anyway, Pausanius does speak of a shrine in Greece, a cave in which there was an unnamed mare-headed goddess. She was supposedly the daugther of Ceres, resulting from Her rape by Neptune as She searched for Proserpina. I think, if I remember right, that Pausanius said this daughter of Ceres had no name, too ashamed ever to leave the cave.

Epona is listed in some of the Fasti, the only Gaulic deity adopted into the Roman cultus civile. I will have to check which for a clue as to when this may have happened, but the assumption is that Epona was accepted in Rome long before any of the Fasti, while the Gauls still inhabited northern Italy. That is sometime between the fourth and first centuries BCE. I wouldn't know, but that may be why, too, that you have non-Celt Roman units given Epona as the protective goddess of imperial cavalry units. All but one of the Fasti date to the Augustan Restoration or later.