Read About Choosing Horse Weight Gain
Horses that are underweight due to illness or other conditions can safely gain weight with specialized supplements.
Things to Consider:
Deworm -- Make sure that your horse has been dewormed, as this is often a reason for weight loss.
Balanced Meal -- Sometimes grains and hay are not enough to produce weight gain for your horse. Adding supplements to your horse's diet will provide an easy solution and effective results.
Stress -- Horses often experience weight loss when they are under large amounts of stress and are unable to eat their normal quantity of food. Small supplements can be given that produce good results.