fictional animal vs beast

fictional animal and beast both are nouns.

Word	Noun	Adjective	Verb	Adverb
fictional animal	Yes	No	No	No
beast	Yes	No	No	No

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

fictional animal: animals that exist only in fiction (usually in children's stories)
beast: a living organism characterized by voluntary movement

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

fictional animal (noun)	beast (noun)
animals that exist only in fiction (usually in children's stories)	a living organism characterized by voluntary movement
	a cruelly rapacious person