List of characters in the Camp Half-Blood series
Hydra - A monster with nine heads. Every time a head is chopped off, two new heads grow back. This monster appears in The Sea of Monsters where its life force is linked to the Monster Doughnut Shop as another one pops up every time the Hydra regrows a head. In The Lightning Thief film, the creature takes the form of five janitors in the Parthenon replica in Tennessee and refers himself as "We" and is ultimately defeated by the use of Medusa's head.