This is excellent work! I love the coloring on the horse, and the detailing of the musculature; you make me ashamed of my own centaurs. ;)
The only critique I can think of is that the tail looks a bit unnaturally brushy--horse's tails have a little more flow to them in my experience. But then, that might just be the fact that his tail matches his hair . . . pardon me if I'm being obtuse.