Your sense of porportion is very good and the rearing horse is balanced very well. He looks like he really is over his center of gravity and not likely to fall over. That can be tough to draw sometimes.
Try some shading. Also, really LOOK at a horse's hoof and pasturen, fetlock and such. Draw just what you see, not what you think it SHOULD look like. Throw away preconcieved notions and draw it exactly as you see it in real life.
Good luck and keep drawing!