Hydra vs mythical monster

Hydra and mythical monster both are nouns.

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

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

Hydra: (Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones
mythical monster: a monster renowned in folklore and myth

Other hypernyms of Hydra include mythical creature.

Hydra (noun)	mythical monster (noun)
a long faint constellation in the Southern Hemisphere near the equator stretching between Virgo and Cancer	a monster renowned in folklore and myth
(Greek mythology) monster with nine heads; when struck off each head was replaced by two new ones