Hi Scott
I was always lead to believe that the horse was late Iron Age/Romano, there are similar depictions of horses on coins of the time, particularly of the Atrebates tribe who were around this area - although so were the Dubonni and Catuvellauni I think. The prominent position on the side of the hill suggests to me that it may have been some kind of tribal emblem designed to be seen from a distance, a sort of ‘we live here’ warning to other groups.
I’ve no idea if it has changed shape over the years though, it was first recorded in the 11th century, so there must be some early illustrations of it somewhere.
Got to agree with you though, it’s a mighty fine beastie.
-Chris