The standard modern fantasy clichés are a combination of Tolkien’s writings and several elements of Greek mythology, particularly in Dungeons and Dragons, where every mythical monster is pressed into service as a possible encounter. Of these immigrants, the hydra has managed to become one of the most famous, appearing everywhere from film to games (both video and card) to television. There is even a genus of cnidarians named Hydra. I suspect the hydra’s popularity might be because it’s an interesting and distinct monster with an interesting ‘gimmick’, evident in its original incarnation.
The original hydra, which was another of the many monstrous offspring of Echidna and Typhon (like the chimera, who was previously featured in this list), lived in the lake of Lerna, in Greece. It had seven heads, one of which was immortal. The slaying of the hydra was the goal of the second labour of Herakles (also known by his Latinised name, Hercules). Herakles and his nephew, Iolaeus, encountered the hydra and attempted to slay it. However, each time Herakles chopped off a head, another two sprouted in its place. Confronted with the impossibility of this task, Iolaeus (inspired by Athena putting ideas in his head) realized that if the stumps of the necks were cauterized (i.e. burnt), they would not regenerate. Armed with this knowledge, Herakles defeated the hydra, and dealt with the immortal head by burying it under a big rock. While the hydra was no more, its poisonous blood reappears in the later stories of the myth of Herakles, used to kill the centaur Nessus and then later to kill Herakles in revenge for Nessus’ death.
There are many multi-headed monsters, but the hydra’s regenerative abilities make it stand out from the crowd.