Yes, horses can founder on apples. Founder is an equine metabolic disorder caused by the overconsumption of carbohydrates, which can occur when large amounts of sugar-rich foods such as apples are consumed. While small quantities of sweet treats like apples may be safe for some horses, it's important to take into account factors such as age and activity level before feeding them any type of fruit or sugary snack. Overfeeding or sudden introduction of large amounts of sugars in a horse's diet can cause laminitis - a painful condition that results from inflammation around the hoof - leading to potential lameness and long-term complications if not treated promptly. Therefore it is best to feed your horse limited amounts of any type food high in sugar content, including apples.