Hydra vs mythical creature

Hydra and mythical creature both are nouns.

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

As nouns, mythical creature is a hypernym of Hydra; that is, mythical creature 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 creature: a monster renowned in folklore and myth

Other hypernyms of Hydra include mythical monster.

Hydra (noun)	mythical creature (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