Observational fieldwork, totaling 1400 hours, was conducted on two species of horse (Equus caballus and Equus ferus przewalski) living under natural social conditions to determine whether lateralization is a characteristic of the species. Previous research on domestic horses has shown sensory as well as motor biases but human influence may have generated such biases. We addressed this by studying eye and limb preferences in two groups of Australian feral horses (N=76) in remote localities and Przewalski horses in a research station in Le Villaret, France (N=33). The latter were chosen because they are considered to be the closest living 'wild' relative of the domestic horse. Both feral and Przewalski horses displayed left-eye population biases in agonistic responses within harem bands and during stallion fights, and also in vigilance and reactive behaviour. Greater leftwards bias was found in more aggressive and highly reactive responses. Przewalski horses displayed greater laterality than feral horses in aggressive responses within harems, and in vigilance and reactive behaviour. Hence, as in other vertebrates, the right hemisphere controls agonistic behaviour and responses to potential threats. In contrast, no population biases were found for limb preference, which indicates that limb preferences reported in domestic breeds may have been induced by selective breeding or handling by humans. Whereas eye preferences are a robust characteristic of horses, limb preferences are variable and modified by experience. Lateralization is, therefore, a characteristic of the horse with eye bias rather than limb preference being be a more reliable indicator of such lateralization.