Anytime a horse is on a diet, treats should be used in moderation. Carrots and apples would be better for treats than candy, horse treats, and cookies. However, apples and carrots still contain carbohydrates and if given in excess can contribute to weight gain. If you have a horse that is overweight and also has insulin resistant (Equine Metabolic Syndrome), even these treats should be curtailed. This horse still needs a proper diet to meet all of the horses nutrition needs, you need to just cut the amount fed.