mythical monster vs monster

mythical monster and monster both are nouns.

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

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

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

Other hypernyms of mythical monster include mythical being.

mythical monster (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