The art is wonderful, and the rider looks great as well, however...
I've never seen such a large muscle on the cannon bone of any horse before (real or imaginary). The muscle isn't even that important but when it leads to such a small foot, it seems rather unnatural.
Also, I don't know if this was intentional, but the unicorn's forearm might look better if you flip the muscle so that it is bulging on the top rather than the underside to match a horse's (like our biceps). As it is, it looks very similar to a man's leg.
Altogether it is a very nice piece, and I merely offer this advice to add more realism to an already wonderful work.