hellhound vs mythical creature

hellhound and mythical creature both are nouns.

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

As nouns, mythical creature is a hypernym of hellhound; that is, mythical creature 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 creature: a monster renowned in folklore and myth

Other hypernyms of hellhound include mythical monster.

hellhound (noun)	mythical creature (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