According to Greek mythology, every time one of this dragon-like monster's heads was cut off, two more grew in its place. It was finally killed by Hercules. What was it?

The creature described, a multi-headed, dragon-like monster from Greek mythology whose heads regenerated in greater number when severed, is indeed the Lernaean Hydra. This formidable beast was one of the most challenging adversaries faced by the hero Hercules, forming the second of his legendary Twelve Labors. Its lair was the swamps of Lerna, a region in the Argolid, which was also believed to be an entrance to the Underworld.

The Hydra was no ordinary foe; its breath was poisonous, and its blood so virulent that even its scent could be deadly. Hercules initially struggled against the monster, finding that for every head he cut off, two new ones would sprout in its place. It was only with the clever assistance of his nephew, Iolaus, that Hercules was able to overcome the Hydra. As Hercules severed each head, Iolaus used a firebrand to cauterize the stumps, preventing new heads from growing.

The Hydra also possessed one immortal head, which Hercules ultimately had to cut off with a golden sword and bury deep under a massive rock. After defeating the creature, Hercules dipped his arrows in the Hydra's poisonous blood, making them incurably deadly. This epic struggle against an endlessly regenerating monster cemented the Hydra's place in mythology as a symbol of overwhelming, persistent challenges.