adult vs fauna

adult and fauna both are nouns.

Word	Noun	Adjective	Verb	Adverb
adult	Yes	Yes	No	No
fauna	Yes	No	No	No

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

Other hypernyms of adult include animal, animate being, beast, brute, creature.

adult (noun)	fauna (noun)
any mature animal	a living organism characterized by voluntary movement
a fully developed person from maturity onward	all the animal life in a particular region or period

adult (adjective)	fauna (adjective)
(of animals) fully developed
designed to arouse lust