Horses come from a long line of prey animals in the family equidae and sleeping on their hooves is an adaptive measure against predators. Standing gives them a literal jump on predators and a better chance of getting away than if they were lying down. That according to Karen Waite, who is an equine specialist at the Michigan State University. Simply put, horses are big animals and it takes them time and energy to get off the ground. A standing horse is therefore much more able to run away when aroused from sleep