bronco (n.)

also broncho, "untamed or half-tamed horse of the American Southwest," 1850, American English, apparently from a noun use of Spanish bronco (adj.) "rough, rude," originally a noun meaning "a knot in wood," perhaps from Vulgar Latin *bruncus "a knot, projection," apparently from a cross of Latin broccus "projecting" (see broach (n.)) + truncus "trunk of a tree" (see trunk (n.1)). Bronco-buster is attested from 1886.