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).
Saddle trees built for a mule have less rock in the bars than a saddle tree built for a horse. The rock of a tree is important because if it doesn’t match the conformation of the horse or mule, the saddle will not be able to distribute weight properly, will move around, and cause soreness.
For instance, if a tree with a lot of rock is placed on a mule with little or no rock, the tree may actually rock on the back, like a rocking chair, this can cause moving pressure points that are very uncomfortable for the mule. Conversely, if you placed a mule tree on a typical horse's back, the tree would bridge in the center of the bars, meaning it would only make contact in the front and back of the bars and would float in the middle of the back. This creates too much pressure at the front and back of the saddle and can cause damage to the horse's back.
Although most mules have a straighter back, some will inherit their back from the horse parent. Likewise, there are horses that have a straight, mule-like back. Due to this, we recommend that each individual should be evaluated before deciding if a mule or horse tree will be the best fit.