First off this piece is stunning. The horse, although the main figure, is not the complete centre of everything. Your eyes are able to wonder around the image, rather then being completely distracted by the main subject.

The lighting is fantastic. You can tell where it comes from not only by the shadow but the light you've added into the trees definitely adds to it. That shadow... It really shows how much attention you put towards details.

I do have two minor details to pick on...
I think the mane is a little thin and a bit sharp. If you thicken it a little bit, it may add to the image.

Secondly, after I strayed my eyes from that gorgeous horse of yours, immediately I noticed the tree to the right. If you darken it just enough, and then add the branches, I think it could really draw your eyes to a different part of the background, rather then seeing that first hand and then scanning around the image.

Overall very well done. I really love this image.