If your horse is too fat, you need to find a way to cut energy (calorie) intake (and/or increase energy output via exercise). Fat loss really is as simple as this. If the horse takes in less energy that he needs, he will use up fat stores. Of course the theory is simple, but its not so easy to implement, in real life. You can look up how to get your horse to go into energy deficit and lose weight by searching for weight loss.