The products that are sold to give your horse extra shine and act as fat and weight builders are often overpriced and filled with fillers. Here's a product that will do both of these things, but you can buy it by the gallon at your grocery store for around $10.
Tip: Adding corn oil to your horse's diet will provide him with more fat and energy, without increasing his excitability. It's also an easy way to up their calorie intake without having to increase the amount of grain they consume. Corn oil provides essential fatty acids, mostly Omega-6 and some Omega-3, which is necessary for absorbing certain vitamins and is great for their coats and hooves.
Another bonus is that horses usually like the taste and it helps keep the dust down in dry foods and help prevent any respiratory issues.
Note: It's important to talk with your vet before adding anything to your horse's routine. Ask them for proper dosage and make sure they think it will benefit your horse.