Euripides

Heracles 1274-5 (c.414 BC)

An early literary reference to the Hydra.

τήν τ᾽ ἀμφίκρανον καὶ παλιμβλαστῆ κύνα
ὕδραν φονεύσας μυρίων…

DSS no. 24. Translation can be found here.