Horses have evolved over millions of years from animals about the size of dogs to the much larger size they are today. The origins of their association with people are unknown, but evidence suggests that they were first domesticated by nomadic Middle Eastern tribesmen around 2000 bc, or even earlier by the Chinese (3500 bc). Unlike dogs and cats, which are predators, horses are prey animals. They feed on grains and grasses and, like all prey animals of open grasslands, they tend to herd together for protection and take flight in response to danger or any unsettling circumstance.
Horses in the United States are not generally raised for meat, hides, or milk. Rather, horses are used primarily for pleasure riding, showing, racing, and pulling carts or carriages. Horses are still used for agricultural purposes (for example, pulling plows) in some areas and by some cultures such as the Amish in Pennsylvania or central Illinois. A wide range of horse varieties and breeds have been created to perform these various functions.