leviathan vs mythical monster

leviathan and mythical monster both are nouns.

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

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

Other hypernyms of leviathan include mythical creature.

leviathan (noun)	mythical monster (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