cockatrice vs mythical monster

cockatrice and mythical monster both are nouns.

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

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

cockatrice: monster hatched by a reptile from a cock's egg; able to kill with a glance
mythical monster: a monster renowned in folklore and myth

Other hypernyms of cockatrice include mythical creature.

cockatrice (noun)	mythical monster (noun)
monster hatched by a reptile from a cock's egg; able to kill with a glance	a monster renowned in folklore and myth