I'd first like to say that your art is one of my favorites. I love this particular piece alot!
I'd like to point out a few things that I felt stuck out to me the most. Here we go.
I'm not sure if they were done with a brush, or if they were hand painted. They seem a bit odd for me. Perhaps add a few more, or take them out all together. They stick out a bit to much, if you are going to have trees in the background, perhaps add a few more so each one doesn't draw too much attention.
The mane isn't so bad, but the tail is slightly stiff. A few finer hairs are in order to keep it from looking like a wooden board, lol. I do, however, love the shading on the tail and mane.
Some Shading work:
In some places, it seemed to be to light in some places. I would try to avoid having colors that close to pure white. It doesn't look like the light source is very heavy, and therefore there is no need for such light areas. The dark shading is perfect though.
The face is a little off. It's a little big for the rest of the body. It would look better if it wasn't quite so large.
Great concept, and you did a great job on this! It was a great idea, and you acheived the idea great! I love it, in my favorites it shall go!