Nice! I think you've got the idea of shading all down. About anatomy, (I have trouble with it all of the time, so I'm no expert :S) what you could try to do, if you want to be exact, is learn the actual structure of a horse, like it's bones and muscles (then, it'll be easier to you know...at muscles to your horse). You did it a lot well then I could, maybe make the horses head a bit smaller. ;)
I'd love to see more soon. ;D
