I like the head of the pulling back horse, too. YOu could put even more feeling in it by having the mouth "tight" and downward angling (displeasure and stress).
That back is a bit long, but don't go erasing the whole thing for that. To me, it's not overly fat. Actually, I like your sense of porportion. Many people don't have that, and youve got the distances from one joint to another just right. As you draw more, you cann exxagerate a bit, like put a bit more "elbow" into it, (that's a joint, too) and show the point of the stifle bone. (a problem in the piaffing horse. his stifle is wrong, too rounded and too low). YOur hooves are nicely drawn, too.
The pulling back horse would probably "splay" his legs somewhat, at least the back ones. So, the hooves would point outward a bit and the hockes inward a bit. Little things like that add believeability.
However, you have really tried to capture a "moment", and this is a lot more challenging that just drawing a stationary horse. A great effort!