There are several reasons as to why a horse might be skinny such as digestive problems, teeth issues, parasite overload, metabolic issues, or ulcers. Most people think the best way to put weight on a horse is by feeding them lots of grain. This is not the best way.
While grain may help put some weight on a horse it is not a healthy way to do it. Grain is for mares that are in foal, young growing foals or horses that are being worked hard. Horse's that are inactive only need good quality hay. If you have poor quality hay you cannot feed them enough grain or other additives to help them gain weight.
If you are feeding your horse plenty of good quality hay and they are still not gaining weight then check the following things.
Digestion - this is the first place to start with a skinny horse. If the PH level is off in the gut they will have a hard time assimilating their food. They may need some extra help with the use of a good pro-biotic to encourage the growth of beneficial bacteria in the gut.
Dental problems – if the teeth have sharp edges it can cause ulcerations on the cheeks and tongue. Teeth problems can interfere with their chewing ability.
Parasites – if there is a parasite overload this could be the cause of the problem. Only deworm as needed and use natural methods instead of chemicals if possible to avoid liver toxicity. If using a chemical wormer go with something that has low toxicity.
Some horse's will lose weight when they go into training because of the stress. If the horse is in the field with other horse's and they are at the bottom of the pecking order this can cause their digestion to shut down causing them to lose weight. Again, a good pro-biotic can help.
If your horse is at a boarding stable do you know how the employees treat the horse when you are not around? Employees that stress the horse out can do a lot of emotional damage to a sensitive horse which shuts down the digestion.
As horses age they can require more feed. The older horse can have a hard time chewing hay. To help the older horse out you can give them extra calories by soaking some hay pellet's.
If all of the above are good then the horse may have metabolic issues. Getting the liver balanced and detoxed will also help with digestion. If the liver is toxic it can cause poor digestion and weight loss. Ulcers are another thing that can sometimes cause a horse to lose weight or not gain weight.
When we are around the horse every day it can be hard to tell if they are losing weight. Get in the habit of checking them with a weight tape every other month. If you decide to change your horses feed make sure you do it gradually over several days to avoid further digestive problems.