Palaephatus, Ovid, and Diodorus Siculus concur with Euripides, while Servius has the Hydra grow back three heads each time; the Suda does not give a number.

Depictions of the monster dating to c. 500 BC show it with a double tail as well as multiple heads, suggesting the same regenerative ability at work, but no literary accounts have this feature.

In particular, Sumerian, Babylonian, and Assyrian mythology celebrated the deeds of the war and hunting god Ninurta, whom the Angim credited with slaying 11 monsters on an expedition to the mountains, including a seven-headed serpent (possibly identical with the Mushmahhu) and Bashmu, whose constellation (despite having a single Head) was later associated by the Greeks with the Hydra.

Upon reaching the swamp near Lake Lerna, where the Hydra dwelt, Heracles covered his mouth and nose with a cloth to protect himself from the poisonous fumes.

He shot flaming arrows into the Hydra's lair, the spring of Amymone, a deep cave from which it emerged only to terrorize neighboring villages.

Heracles placed the head—still alive and writhing—under a great rock on the sacred way between Lerna and Elaius,[9] and dipped his arrows in the Hydra's poisonous blood.

Hera, upset that Heracles had slain the beast she raised to kill him, placed it in the dark blue vault of the sky as the constellation Hydra.

Heracles would later use arrows dipped in the Hydra's poisonous blood to kill other foes during his remaining labors, such as Stymphalian Birds and the giant Geryon.

Greek and Roman writers related that Hera placed the Hydra and crab as constellations in the night sky after Heracles slew him.