Four hundred years is a pretty long tradition in our ever changing world. Add Tysoe in the 1600's and Gogmagog in Plymouth in the 1400's to push the tradition back further.
If this is a horse carving, then it's a first on flat land and without precedent in these islands.
You also have to consider what else it could be. It reminds me of the almost annual reports of people seeing the face of christ in a stain on the pavement or the name of allah written on the side of a fish - you see what you want to see.
To me a rational way of looking at things is to use common sense.
It's a mark on a field that vagely resembles a horse - until proved otherwise.