Hydra

The Hydra is a vicious multiple-headed monster.

History
In Greek myths and legends, the Hydra was a vicious monster with multiple heads with one of them being immortal and invincible to all weapons. When one head was decapitated, two more snake-heads would grow in its place. Hercules eventually defeated the Hydra, by having his nephew, Lolous, burn each stump after he had cut it, before the heads reproduced, then tore off the immortal head with his bare hands, and buried it under a giant rock.

The Sea of Monsters
When Percy Jackson sent Tyson to look for powdered doughnuts so he could talk to Annabeth, Tyson went out and came upon a Monster Doughnut store. Tyson, being a monster, was somehow able to summon the outlet bringing the Hydra with it. It was large and lizard-like, with long necks and seven heads to match. It had many rows of razor-sharp teeth, its hands were clawed, had a tail, and spat acid. According to Annabeth, another Monster Doughnut shop popped up every time it grew another head. She says that Hermes' children found a way to use the hydra's life-force to make a new chain of stores. Percy, initially tries to defeat it by slicing off its heads, but succeeded only in making more. Percy, Annabeth, and Tyson were saved by Clarisse when the Hydra's heads were lit aflame and burnt off by cannon fire from the CSS Birmingham.

The Mark of Athena
Percy fights a hydra when he, Piper McLean and Jason Grace arrive to rescue Nico di Angelo from the giants Ephialtes and Otis.

Spoil of War
In the attic of the Big House in Camp Half Blood, the original Hydra head was left as a spoil of war. This incarnation of the monster was defeated in Woodstock, New York in 1969.

Powers and Abilities

 * It can spew acid from its mouth.
 * Some heads have the ability to breath fire.
 * It has multiple heads
 * Two heads will grow back for each head that is cut off.

The Lightning Thief
Percy Jackson, Annabeth Chase, and Grover Underwood encounter the Hydra in the form of five Security Guards at the replica of the Parthenon in Tennessee, after Percy Jackson takes Persephone's Pearl (originally Poseidon's in the book) from Athena's statue. When Percy cut off the Hydra's Head with Riptide, two more appeared when Annabeth was too late to warn him. Grover managed to defeat the Hydra using Medusa's head - a possible reference to some accounts of Perseus slaying Cetus. It should be noted that the Hydra is depicted as breathing fire in the film instead of acid. After Percy cuts its middle head, two of the Hydra's head breathe fire.

Trivia

 * Hydra, the largest of the modern star constellations, is named after the monster.
 * Hydra is also the name of a satellite of Pluto.
 * Hydra, a species of simple fresh-water animal, is named after the monster.
 * Hydra, is also a name for a small problem.
 * In Greek Mythology, the Hydra terrorized the Kingdom of Lernea, so it is fully called the Lernean Hydra.
 * In one version of Heracles's labors, Hera's first choice of monster to make Heracles fight was the Hydra but Iris tricked her into making the Nemean Lion the first labor, and told Heracles to use the pelt of the lion as armor against the poisonous Hydra,