Nice!
I like how it looks. I have few suggestions though:
- the mane and tail should be a bit more dynamic, from what I see the horse is running so thy should blow more in the wind
- there are 2 main problem with the lighting in my opinion. From what I see the light source is on his/hers right so the drop shadow should be on the left (not top down) and should inherit a silhouette from the horse, and 2, I don't quite see the shading on the subject (but I am color blind and only I can't see it... I do see some on the right side but I think it should be more
All in all... looks nice keep it up :D
P.S. I hope this was helpful for you, sorry fro my bad English