A migratory, grassland mammal (family Equidae) that is adapted for running fast. It has a short humerus and femur, long radius and tibia, the ulna reduced and fused to the tibia, and only the third toe developed. The brain is large and the senses highly developed. Horses are social, with complex social organization. They evolved mainly in the New World, but became extinct there until reintroduced by Europeans in the sixteenth century. There are six or seven species, all in the genus Equus, including the domesticated horse (E. caballus), Przewalski’s horse (E. ferus przewalskii), asses, and zebras.