A horse under pad is a cushioning layer placed between a horse's back and the saddle to provide extra comfort and protection during riding. It is used to prevent rubbing, chafing, and pressure points that can cause discomfort and pain for the horse. The under pad is typically made of materials like fleece, wool, or gel, and is designed to help distribute the weight of the saddle more evenly and absorb impact while riding. It is an important accessory for ensuring the horse's well-being and comfort during equestrian activities.