hellhound vs mythical monster

hellhound and mythical monster both are nouns.

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

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

hellhound: (Greek mythology) the three-headed dog guarding the entrance to Hades; son of Typhon
mythical monster: a monster renowned in folklore and myth

Other hypernyms of hellhound include mythical creature.

hellhound (noun)	mythical monster (noun)
a very evil man	a monster renowned in folklore and myth
(Greek mythology) the three-headed dog guarding the entrance to Hades; son of Typhon