If you can count all of the ribs on your horse, his hipbones and shoulder bones are sticking out and there are dark, concave areas above his eyes, then he is way too thin. Since horses have such an insatiable appetite, any loss of appetite or weight loss should be viewed as a danger sign that your horse is sick.
Heaves is a respiratory disease that causes the horse to exhale twice for every inhalation. The horse usually has a low cough that starts whenever he enters a dusty place or is given dry hay. When you put your head to his chest, he may give off a wide array of strange wheezes and rattles. The problems in breathing tax the horse's body, causing him to lose weight. This is a manageable condition under veterinary care.
All horses are prone to getting internal parasites like heartworm, tapeworms and bots, no matter how clean the barn is. This is because many horses catch worms from biting flying insects, which are hard to eliminate entirely. Heavy infestation of worms can cause weight loss, hair loss, anemia and damage to the internal organs. This is why it is important to regularly de-worm your horse.
Cushing's Syndrome is a disease of the horse's endocrine system that makes it overproduce the stress hormone cortisol. This constantly stresses the horse, making them lose weight even as their appetites increase. They also will drink more (and thus urinate more) and their hair will become shaggy, rough and sometimes even curl. This is mostly seen in older horses.
This is another condition that effects older horses, but does occur in younger horses Any tumors in the stomach or intestines there for any reason (including cancer) are referred to as neoplasia. These usually need to be surgically removed in order to help the horse survive.
This is particularly cruel disease in that the horse is suddenly allergic to grass and hay--their principle foods. They gradually starve to death. This is the disease that killed Dubai Millennium, winner of the Dubai World Cup in 2000 at the young age of five. Not much is known about grass sickness. It began in northeast England in the 1970's and has gradually spread throughout the world.
Weight loss is a common symptom of any kind of liver disease in a horse. This usually comes on all of a sudden (sometimes only three days). Other symptoms include diarrhea, colic, fever and mouthing food but not eating it. Many horses will also circle around aimlessly and will try to hide from any light because it hurts their eyes. Treatment needs to be immediate to try and save the horse.