I really like what you've done.
I think the biggest point to improve on would be the crispness of your "outline." The edge of the horse is very "wavy." One of the artists around here came up with a method to "mask" your subject and paint the background. Sketch your "line art" onto the paper (acrylic painting paper, etc.), and then use masking tape to cover it. Use a box cutter (or similar artist knife) and cut off the excess tape so the tape covers your subject. Paint the background, then carefully remove the tape an paint the subject.
It's really a neat trick and I've only played with it. Regardless, you have amazing shading skills and I love how you blended colors (you didn't use just
brown, gray, black, etc.). The muted background also draws focus to the horse. Great job!