Wowee, first off, this is excellent. Even the better you got paid for it! The pose is very excellent, so is the expression and the contrasting colors. You did very well here.

Though, there are minor changes that can be made to make it better. Firstly, when I look at the peice, my eye is drawn to the downward line of the barrel. Horse's bellies curve upwards, even if the horse is standing on a slope. Since the brilliant red is on that one leg, my eye also sees the knee there. It's a tad bit big, and the leg around it is a little small. Think of the knee as rising up out of the leg, instead of the leg getting skinnier before the knee.

The last, but definitely easily fixable thing, is the left front hoof. The leg tapers down to this little hoof, when in actuality the hoof should be about the same thickness as the leg, depending on the breed of horse. Try and make the heel of the hood longer, so that it has "four sides" including the hairline on top, instead of a triangle.

But overall, great, great work. You captured emotion, a good pose, and even shaded dirt well (lord knows I can't). Good job.