adult vs beast

adult and beast both are nouns.

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

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

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

adult (noun)	beast (noun)
any mature animal	a living organism characterized by voluntary movement
a fully developed person from maturity onward	a cruelly rapacious person

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