bronc vs bucking bronco

bronc and bucking bronco both are nouns.

Word	Noun	Adjective	Verb	Adverb
bronc	Yes	No	No	No
bucking bronco	Yes	No	No	No

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

bronc: an unbroken or imperfectly broken mustang
bucking bronco: a wild horse that is vicious and difficult or impossible to break in

bronc (noun)	bucking bronco (noun)
an unbroken or imperfectly broken mustang	a wild horse that is vicious and difficult or impossible to break in