Have you ever seen a really fat Mustang waddling free on the wide open prairie? Horses are not meant to be obese. They are designed to be running machines in order to race away from predators and to travel many miles in a day for foraging. They were not designed to stand in a stall or pasture and munch for most of their lives.
As a result, many domestic horses are far too fat. In a study of 300 random horses done in the summer of 2006, a research team from the Virginia-Maryland Regional College of Veterinary Medicine found that 51% of the horses they saw were way too fat. This sends horses on the fast track to many health complications and a shortened life.
Insulin Resistant Diabetes
Just like obese people, obese horses can get potentially lethal problems regulating their blood sugar. It's thought that horses never contracted diabetes until they were introduced to high-fat concentrated feeds and didn't get the exercise their ancestors did. Equine diabetes acts very much like human Type II diabetes.
Diabetic horses aren't merely prone to going into a lethal diabetic coma when they are obese, but they are also far more prone to contracting hoof diseases. Hoof diseases are devastating for a horse.
Laminitis
Perhaps one of the most famous horses in the world, Triple Crown winner Secretariat, died as a result of laminitis. He had been considered fat even as a racehorse there was a reason they called him "Big Red" after all but he absolutely ballooned when retired to stud at age three.
He was coddled for the rest of his life, only having to perform his studly duties every now and then. He was estimated to be about 1,000 pounds overweight when he developed laminitis and had to be put to sleep because of the constant incredible pain he was in. It is now thought that all of that could have been avoided and Secretariat could have lived years longer had he been kept on a proper diet and given regular exercise.
A study done in the Veterinary School at the University of Queensland suggests that laminitis is caused in part by glucose changes in the hoof's laminae which happens when a horse has trouble with his blood sugar as a result of being far too fat.
Heart Disease
It's true in all mammals if you are too fat, your heart has to work harder. You also tend to develop plague in the arteries which could lead to a coronary. Horses are no different. If your horse is also diabetic, then he may already have circulation problems, which the heart tries to work harder in order to compensate.
Clearly, if you have a heart, then you will work with your vet to make sure your horse is getting a good diet and exercise in order to live a long and pain-free life.