[header] How to fatten up a horse [title] Check your horse for health issues. [step] Before you begin fattening up your horse, make sure there are no other health problems that could be causing weight loss. You'll need to take your horse to the veterinarian to ensure your horse is healthy. 
If your horse has a medical condition, the veterinarian will be able to prescribe the correct treatment, which will most likely help your horse regain any lost weight. [substeps] The two major causes of weight loss in horses are internal parasites (worms) and poor dental conditions.