When asked about their horse's diet most horse owners want to tell me about the pelleted feed the horse is getting. The pelleted feed should be an afterthought in most cases. The typical way we feed horses is abnormal for their physiology. They are meant to munch on low quality grasses for about 18 hours a day. For convenience, horses are often placed in a stall and fed only 2 or 3 meals a day with nothing in between for hours at a stretch. This is the way carnivores eat- not horses.
When the horse has hay to munch on it is better for them. When they go without food, stomach acid begins to build up. This can irritate the stomach lining and lead to ulcers, especially with horses that are a bit anxious. Eating hay calms their minds as well as their stomachs.
Hay is cheap sedation. In the rescue course I took, the instructor stressed this point in the odd context of dealing with trailer accidents. Dr. Tomas Gimenez taught that one of the first things that should be done for the horses is find some hay and throw it in the trailer, even if it is mangled and on its side. The horses will go right to it, and it settles their minds. The horse that is in the stall for several hours without hay is probably worrying about when, if ever, he is going to see the next flake. This practice is probably the origin of cribbing and stall weaving.
It also makes no sense to let a horse go without hay when trying to maintain their weight. A typical scenario I encounter is an owner is trying to get their horse to gain weight. They are feeding lots of pelleted feed, often switching from one to another to find that special one that works. They are also feeding supplements because they are convinced their horse has to be deficient in a mineral or an amino acid. Yet, the horse goes several hours a day without forage. Feeding like this is not good for your horse, and most supplements do nothing but make your wallet thinner. I would prefer that your horse has all they hay* it wants, and if it cannot maintain its weight on the hay alone, then increase the pelleted feed. This will be better for the horse in the long run.
*Just a note about Coastal Bermuda hay- there does seem to be an association of feeding Coastal Bermuda hay with colic. See the paper here .
But what about the horse that looks at hay and gains weight? Well, this is a challenge for you. The horse is designed to graze most of the day. If you give him free choice hay he will probably develop laminitis. For the reasons mentioned above, putting your easy keeper on a starvation diet it isn't good for him either. The answer- you have to increase the horse's activity level. Keep him moving; ride him more. This will burn off the calories and help his metabolism.
This is a challenge as well because feeding a starved horse incorrectly can kill it. Be careful! These horses should not have free choice hay. Their metabolism is altered when they have been starved. If they get too many calories too soon they can colic, or founder. Many small meals several times a day is the best approach. See this article for more information on re-feeding syndrome.
This can be tough to answer depending on what you do with your horse. Many quarter horses that go to shows are technically overweight. But this is how the judges like to see them- that big butt is the norm now. Many thoroughbreds are technically underweight. You will usually go broke trying to make a thoroughbred fat. For your horse's health we should try to get them as close to a 5/9 on the body condition score system as we can. I look at horses every day and when asked about weight I always break out the chart because our sense of what is normal can drift. We get used to looking at our horse and we just don't notice that something is wrong until we look at the chart. It keeps us honest. Here is a copy.