horseman vs adult male

horseman and adult male both are nouns.

Word	Noun	Adjective	Verb	Adverb
horseman	Yes	No	No	No
adult male	Yes	No	No	No

As nouns, adult male is a hypernym of horseman; that is, adult male is a word with a broader meaning than horseman:

Other hypernyms of horseman include equestrian, horseback rider, man.

horseman (noun)	adult male (noun)
a person who breeds and cares for horses	an adult person who is male (as opposed to a woman)
a man skilled in equitation