The horse is a single-hooved mammal that has evolved over 45-55 million years from a small multi-toed creature into today's large single-toed animal. Horses have been domesticated as the domestic horse subspecies, though some feral populations still exist. Horses have adapted speed to escape predators and have a strong fight-or-flight response. Their gestation period is around 335-340 days resulting in usually one foal. Horses are herbivores with a digestive system adapted to a continual grazing diet of grasses and plants.