A horse pad liner is a type of cushioning or protective material that is placed beneath a horse saddle pad to provide additional comfort and support for the horse. It helps to prevent rubbing or chafing, absorbs sweat, and can also help to distribute pressure more evenly across the horse's back. Horse pad liners are typically made from materials such as fleece, gel, or memory foam, and come in various shapes and sizes to fit different types of saddle pads.