But seriously, if you live around animals that eat grass and run around in a pasture all day, you begin to wonder when they get some shuteye. Its an easy question to answer if you're talking about an animal like a dog or cat because we see them curl up into a comfortable ball and doze off.
Horses are different because they are flight animals.
No, that doesn't mean they have wings. It means they have to be ready to runaway from danger. They don't have thick armor or sharp teeth, so a horse's best means of defense is a good, fast gallop. Taking off from an upright stance is a whole lot easier than having to get up first, and then start running. The answer lies in a system of tendons and ligaments that work together with the knee cap.