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 immortal and invincible to all weapons. When one head was decapitated, two more snake-heads would grow in its place. Heracles 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 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 a long neck and seven heads. 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 and Annabeth were saved by Clarisse when the Hydra's heads were lit aflame and burnt off by cannon fire from the CSS Birmingham.

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.

The Lightning Thief
The Hydra is featured in The Lightning Thief. Percy Jackson, Annabeth Chase, and Grover Underwood encounter the Hydra in the form of 5 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. The Hydra first appeared in The Sea of Monsters book.

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 terrorised the Kingdom of Lernea, so it is fully called the Lernean Hydra.
 * In one version of Heracles's labours, Heras first choice of monster to make Heracles fight was the Hydra but Iris tricked her into making the Nemean Lion the first labour, Iris told Heracles to use the pelt of the lion as armour against the poisionus Hydra