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.