Curly, depends on the mule. They tend to be flatter backed both ways than a horse more like a donkey but a lot of horse saddles will fit a mule. A number of companies make mule trees/saddles but those aren't guaranteed to fit all mules either. The guy I bought my saddle from called it a mule saddle (he bought it from a mule person to use on his horse, which makes no sense to me if he knew it had a mule tree) but hard to tell by looking, it's not semi-QH and it's not full, seems to be somewhere in between.