"Colic in horses is defined as abdominal pain, but it is a clinical symptom rather than a diagnosis. The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract."
Colic is a very real danger to the equine community. It can be deadly to equines (Horse, Donkey, Mule, Zebra etc). Horses are food vacuums, they eat almost constantly to keep their hindgut moving. When they suddenly stop, look listless or in pain and start to look sick; there's a problem. Anywhere from lowering stress during travel to daily feeding activities colic prevention should be on the mind of everyone involved in horse care.
So I listed the most common causes of colic. Of course there are many other ways things can go wrong but having the basics in mind is best.