Hi Martin,
This is a really strong idea and I like it.
Part of the reason I like it is that you do have to ask yourself how this photo was made. Were you lying on the ground to get the shot, and was this horse standing over you, stomping his hoof and threatening you? It is really quite intimidating.
About the only suggestion that I would make to improve this image would be to try and get more detail into the shadows, particularly about the nostrils. This detail does not need to be bright… it just needs to be there to lure us further into the image.
Aside from that, this is a great photo. Well done.
Cheers,
Anthony