centaur vs mythical monster

centaur and mythical monster both are nouns.

Word	Noun	Adjective	Verb	Adverb
centaur	Yes	No	No	No
mythical monster	Yes	No	No	No

As nouns, mythical monster is a hypernym of centaur; that is, mythical monster is a word with a broader meaning than centaur:

centaur: (classical mythology) a mythical being that is half man and half horse
mythical monster: a monster renowned in folklore and myth

Other hypernyms of centaur include mythical creature.

centaur (noun)	mythical monster (noun)
(classical mythology) a mythical being that is half man and half horse	a monster renowned in folklore and myth