I like the background of horses. I wonder if a three quarter slice of the front horse might enhance this image. The eye is well in focus, so it seems. But the mouth and noise are expressive parts which are missing in large part from this photo.
To me, this composition is too much pre-conceived and calculated. But even as it is, I think that the DOF is too shallow, and the picture would look a bit better with the background in focus.
the oof shapes of the horses and trees are really lovely. we see just enough of the foreground horse to identify with him, to wonder if everything else is just taking place in his mind. perhaps those fuzzy shapes behind him are memories...
He seems very shy and like an outcast. He's barely in the frame and away from the others. There's a lot of division going on. The nearby tree acts almost as a barrier to the other horses and the shallow DOF enhances that separation as does the perpendicular line they form. I'm not sure a better composition exist!
I actually LIKE the way you've composed this and the DOF is right on the mark for this. There is enough in the bg for recognition of the subject, but not so much it detracts from the primary (foreground horse head). I find it inspiring enough to try one of these myself. :D