1911 Encyclopædia Britannica/Mustang

MUSTANG, the wild or semi-wild horse of the prairies of America, the descendant of the horses imported by the Spaniards after the conquest in the 16th century (see Horse). The word appears to be due to two Spanish words, mestrenco, or mostrenco, defined by Minsheu (1599) as "a strayer." Mestrenco (now mesteno) means "wild, having no master," and appears to be derived from mesta, a grazier-association, which among other functions appropriated any wild cattle found with the herds.