Sulpicia Ulpius scripsit

The one line in this quote that grabbed me was "Mare-headed Mother" - have any such literal depictions been found, or is she always in the forms you described earlier? COz i would REALLY be interested in that sort of depiction especially if appearing more western Empire way!

This is more of an update than an answer, just to let you know I am still on it. Firstly, I am checking the original Latin text to be sure exactly what it says (the English translation was the one by Graves).

Secondly, there are no known representations of Epona with a mare's head, if we assume that a sculpture of a woman seated side-saddle on a mare, often with a foal, represents Epona sitting on a horse and not Epona with someone sitting on her back. Thats an if, the answer is frequently presumed but rarely discussed, and I am investigating further. There is also an assumption that all types of Epona statuary had an equal chance of survival and that some times were not considered 'more idolatrous' than others and preferentially destroyed in the fifth and sixth centuries.

There is a related question, did Epona start out as a 'divine mare' who became progressively anthropomorphised, first to a woman sitting on a horse and then to a woman seated on a throne attended by horses (this model assumes the Gaulish sidesaddle type is earlier than the Imperial throned type) or did Epona start out as a more generic mother goddess of fertility and abundance, gradually becoming more specialised onto horses (this is argued for by the cornucopiae, baskets or patera of food, nursing mares, and so forth).

Lastly, if there were representations of Epona as just a mare, without humans, without inscription - how would we know and how would we distinguish this type from ay other votive horses which could have been offered to various deities both Romano-Celtic (eg 'the Celtic Mars') and indeed Roman (cf for example the sacrifice of the Equus October).

So, a few interim thoughts pending a full answer.