cay·use \ˈkīˌ(y)üs, ̷ ̷ˈ ̷ ̷\noun (pluralcayuseorcayuses) 1.usually capitalized a.: a Waiilatpuan people of Washington and Oregon b.: a member of such people 2.: the language of the Cayuse people 3.plural cayuses, West: a native range horse; especially: indian pony — compare bronco, mustang