There are saddle pads made for sway backed horses. Skito will custom make shims for their pads based on photos of your horse. The shims are removable, so if the horse gains muscle or you buy a different saddle, the pad can be adjusted as needed. The Skito Correction pad
goes over a blanket or thin pad. The Skito Insert pad
has the option of shims and is used by itself. I really like my Skito saddle pads. They hold up well over the years too.