I must say I'm not usually a fan of cell shading but I must say this turned out wonderful! The horse looks pretty good, in my opinion since you do have rays of light coming in you do need some more highlights especially in the mane and tail. You have the perfect amount of dark shading, it doesn't look overly dark (which I end up doing myself quite a bit ) The shading in the neck is a little weird, not that you put in the wrong place or did it too dark , it just looks a little off the way that it transitions down the the chest , perhaps try to blend that in a little bit better next time. The hooves don't look too shiny, I know a lot people (including myself) put a lot of shine on the hooves, but its really quite un-natural in that horses hooves aren't all that shiny, so thats good. I love the follow in the tail, you can really see the tail bone which really adds a bit of realism in the piece.

As far as the background goes, it looks over all very nice (better than I could do) I only have a few things that I think could improve it, one add a little bit more shading to the trees, two add a few blades of grass over the hooves so it looks like the character is actually walking in the grass (like you did with the trees) and three blur the the trees that are way in the back a bit, this gives you a bit of depth.

Like I said overall very nice pic, good job!