I saw this picture somewhere on the net and was possessed to save it, forgetting about it not too long afterward. You have excellent feel for action and that all four legs are connected to the horse.
Many young artists try for action and forget that bones move underneath all that pizzaz.
The flying mane and tail and even the ears back show the humor of this animal. There a couple of little things you could improve on that would make this drawing really smashing. First of all your basic anatomy is almost right. The shoulder, where the point of the shoulder would be, is a little puny for the horse. Still not bad.