Cerberus vs mythical monster

Cerberus and mythical monster both are nouns.

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

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

Cerberus: (Greek mythology) the three-headed dog guarding the entrance to Hades; son of Typhon
mythical monster: a monster renowned in folklore and myth

Other hypernyms of Cerberus include mythical creature.

Cerberus (noun)	mythical monster (noun)
(Greek mythology) the three-headed dog guarding the entrance to Hades; son of Typhon	a monster renowned in folklore and myth