mustang vs bronco

mustang and bronco both are nouns.

Word	Noun	Adjective	Verb	Adverb
mustang	Yes	No	No	No
bronco	Yes	No	No	No

As nouns, bronco is a hyponym of mustang; that is, bronco is a word with a more specific, narrower meaning than mustang:

mustang: small hardy range horse of the western plains descended from horses brought by the Spanish
bronco: an unbroken or imperfectly broken mustang

Other hyponyms of mustang include bronc, broncho.

mustang (noun)	bronco (noun)
small hardy range horse of the western plains descended from horses brought by the Spanish	an unbroken or imperfectly broken mustang