monster vs leviathan

monster and leviathan both are nouns.

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

As nouns, leviathan is a hyponym of monster; that is, leviathan is a word with a more specific, narrower meaning than monster:

monster (noun)	leviathan (noun)
(medicine) a grossly malformed and usually nonviable fetus	the largest or most massive thing of its kind
a cruel wicked and inhuman person	monstrous sea creature symbolizing evil in the Old Testament
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