The difference is in the “rock” of the trees. The rock is the contour of the bar of the tree, and the back of the horse or mule, from front to back. Typically, mules have a straight back with very little rock. They inherit this conformation trait from their donkey parent. Horses have more contour or sway in their back when compared to a mule. Horses can range from straight-backed (some may use mule trees) to extreme rock (swaybacked).