Bronco

google

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mid 19th century: from Spanish, literally ‘rough, rude’.

wiktionary

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Borrowed from Spanish bronco(“rough”), 19th c. which in Mexican usage also describes a horse that has not been broken and is still wild.

etymonline

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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.