Other explanations are that it's not a horse at all, but that the "fangs" on the face indicate that it's a sabre tooth tiger. This is also unlikely since the earliest dating of the figure only goes back as far as 1000BC.
There are other ancient hill figures: the more famous Cerne Abbas Giant is thought to be Romano British, and the Long Man of Wilmington pagan Anglo Saxon. Although medieval manuscripts contain references to many figures now lost, other remaining hill figures date from the eighteenth to twentieth centuries.