Feeding vegetable oil to horses is an easy way to add fat to your horse's diet. This is perfect for older horses and horses that are hard keepers and need to gain weight. Gradually build up the amount of oil you feed though, otherwise you could cause stomach upset.
Vegetable oil is a highly palatable energy source for horses as well as a coat enhancer. I have found that mixing it with molasses helps horses enjoy it even more, plus making powdered supplements acceptable.