What a pose! A very fun, whimsical look. The legs have a wonderful sense of motion. The right rear could be slimmed down a tad along the upper half. You also could bolster the tail and give it that little shine along the ridge. I think the sharp angles on the face could be brought out even more to give it that Arab look. Similarly, the area around the eye (all those places where the skin is very thin) might be darkened and the nose turned up to further that look. The simplicity of the background is good - not distracting. You might perhaps smooth it out in the distance to create further perspective, though I see you have done something to that effect (thinner lines). The shadow blends in well with the ground, settling into those little dips and curves. All in all, very nicely done!
I agree with Define-X, with the addition of possibly lighten the upper half of the background for contrast since the horse, lower half bg, and upper half have the same tonal plains. I believe it would make the details in the head and neck stand out better, overall this is a well done piece.