mythical creature vs monster

mythical creature and monster both are nouns.

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

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

mythical creature: a monster renowned in folklore and myth
monster: an imaginary creature usually having various human and animal parts

Other hypernyms of mythical creature include mythical being.

mythical creature (noun)	monster (noun)
a monster renowned in folklore and myth	(medicine) a grossly malformed and usually nonviable fetus
	a cruel wicked and inhuman person
	a person or animal that is markedly unusual or deformed
	an imaginary creature usually having various human and animal parts
	someone that is abnormally large and powerful