basilisk vs mythical monster

basilisk and mythical monster both are nouns.

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

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

basilisk: (classical mythology) a serpent (or lizard or dragon) able to kill with its breath or glance
mythical monster: a monster renowned in folklore and myth

Other hypernyms of basilisk include mythical creature.

basilisk (noun)	mythical monster (noun)
small crested arboreal lizard able to run on its hind legs; of tropical America	a monster renowned in folklore and myth
ancient brass cannon
(classical mythology) a serpent (or lizard or dragon) able to kill with its breath or glance