adult vs creature

adult and creature both are nouns.

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

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

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

adult (noun)	creature (noun)
any mature animal	a living organism characterized by voluntary movement
a fully developed person from maturity onward	a living being
	a person who is controlled by others and is used to perform unpleasant or dishonest tasks for someone else

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