Specifically, a non-ruminant browser. In nature, the horse will eat grasses, roots, bark and leaves. With a single stomach and an inability to vomit, everything a horse ingests must pass all the way through his digestive tract. He also can only breathe through his nose; so anything that blocks nasal passages will suffocate a horse.
WATER is the most important nutrient a horse receives. It must be clean and cool in the summer, warmed in the winter. The horse requires free choice salt at all times.
The horse needs protection from the sun, rain, hail and snow. He needs access to wind-break and a safe place to eat (at ground level is best). Food should be in a container and not fed on top of sand. Ingestion of sand can kill a horse.
They require companionship. They are designed to be in motion 20 hours out of 24, grazing and moving the whole time. They do not have gall bladders, the liver secretes bile into the stomach continuously for digestion; they are designed to eat small amounts constantly.
Horses in a herd watch out for each other, mutual groom and stand guard for others to lie down to sleep (that REM, deep sleep is necessary!), taking turns.
Exercise is essential for good health on ALL levels.