mustang vs bronc

mustang and bronc both are nouns.

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

As nouns, bronc is a hyponym of mustang; that is, bronc 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
bronc: an unbroken or imperfectly broken mustang

Other hyponyms of mustang include broncho, bronco.

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