Indeed, it is just a guess. It was made long before we have any surviving written records, and even when we do have accounts of Britain they tend to be very external and not terribly concerned with such things. However, we can compare the image to much later Iron Age representations on coins that are more clearly horses while baring some resemblance to the earlier Uffington depiction. Also, if we assume a basic cultural continuity we can use the later representations as evidence for a high value given to horses, that likely stems from earlier times.

It is also, well, just generally pretty horselike. I suppose it could be a fox or something but I don't really see much reason for denying the general horseyness.