See Cayuse in American Heritage Dictionary 4
noun
Origin:
Origin: Short for cayuse pony
Origin: , from Cayuse
. Regional Note: The noun cayuse comes from the name of the Cayuse people in the Pacific Northwest. Cayuse is used chiefly in the territory of the word's origin—the states of Washington, Oregon, and Idaho—although its use has also spread into other Western states. A verb meaning “to buck,” derived from the noun, is cited by Ramon F. Adams in Old-Time Cowhand (1961): “What cowboys in other sections called buckin', the Texan called pitchin', and a term used in South Texas, though seldom heard in other sections, was cayusein'.”noun pl. cayuse Cayuse or Cay·us·es