I discovered Paul Bonner`s art recently but I have never seen the painting you were inspired from. Any idea where I can find more of his works (except his site)?
The details add a lot to your model`s quality. Unfortunately I also see some problems like the proportions, already mentioned above and the horse`s feet. The front ones have a strange curvature and the muscles look off in most of the places. Also, I think it would be a good idea to model the hair and the tail of the horse in ZBrush.
I hope you don`t mind my observations, I`m trying to help because I like the model.