leviathan vs mythical creature

leviathan and mythical creature both are nouns.

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

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

Other hypernyms of leviathan include mythical monster.

leviathan (noun)	mythical creature (noun)
the largest or most massive thing of its kind	a monster renowned in folklore and myth
monstrous sea creature symbolizing evil in the Old Testament