Colic. It's a word no horse person wants to hear, but laymen often respond with "Colic? Isn't that just a tummy ache."
Horses are unusual amongst large grazing animals in that they are not ruminants. They are what we call "hind gut fermenters."
A horse has a small stomach and a massive gut. The small intestine is 50 to 70 feet long. The large intestine has three sections - the four foot long cecum that actually performs the task of digesting high cellulose food such as grass, the 10 to 12 feet long large colon and 10 to 12 feet long small colon. So, in total, a horse's gut can be as much as 98 feet long!
And, horses are unable to vomit or burp. Any gas that forms in that massive length of gut can only go one way - backwards. Because of this, when a horse gets gas it's much more painful...and long-lasting...than when we get gas. The most common cause of colic in horses is gas, but it's really unpleasant for the horse and we often give them a pain reliever such as banamine. Some horses get gas colic every spring without fail - highly annoying for their owners!
However, because of that vast length of gut and the very complicated system, horses are also prone to "torsion" or a "twisted gut." This means part of the intestine has gotten tangled around another part, which can result in cut off circulation and the death of part of the gut - which is an emergency surgery situation. A twisted gut can easily kill a horse. Because of this, colic is always a veterinary emergency in horses. Colic can also be a result of simple "impaction" - otherwise known as constipation, but it's always desirable to check.
The three other things that can cause abdominal pain in horses are:
1. Sand colic. On very sandy soils, horses can ingest sand along with the grass they're eating, which can then cause an impaction. Psyllium is fed as a supplement to "ball up" the sand and help it pass through the gut.
2. Stomach ulcers. These are seen most often in competition horses and are so common in racehorses that many trainers just give ulcer meds to every horse rather than checking for them.
3. Female symptoms. Mares in heat can get, well...that thing so many human women dread about "that time of the month."
A horse with colic will tend to be off their feed, they will be restless and may kick or look at their sides. They might pace, lie down and get up repeatedly or roll. They often also sweat up. Note that this behavior is also seen in mares about to give birth.
Traditionally, horses with colic were walked because it was believed that would help remove any impaction and because it was (wrongly) believed that a twisted gut was caused by a colicky horse rolling. These days, there's some disagreement on that, but if you're writing anything set before about the 80s or in a secondary fantasy world, then walking a horse with colic would probably be standard.