Answers (2)
Horses are prey in the wild and as you should know they are large creatures. When horses lay down it is hard to stand up. 1 horse will stay on watch and if there is danger they can all run fast.
Horses stand whilst they sleep because In the wild they'd generally be hunted!so if they stand whilst sleeping then it allows them to obviously sleep&have a quick get away if needed!from pray!
Horses are prey animals, so they are hunted by predators in the wild. If they stand while they sleep, they have a better chance of surviving an attack than if they had lain down. Horses have a fight flight or freeze defense, but they usually choose the flight defense. If they sleep while standing, horses have a better chance of waking and out running the predators during an attack.
Because horses can stand still without falling over. Just like cows :)