Ladon

Ladon (Λάδωνας in Ancient Greek) is a massive dragon that has a hundred heads. He guards the Garden of the Hesperides.

History
Ladon was the serpentine dragon that twined and twisted around the tree in the Garden of the Hesperides and guarded the golden apples. He was beaten by Hercules. The following day, Argonauts passed by, on their chthonic return journey from Colchis at the opposite end of the world, and heard the lament of "shining" Aigle, one of the Hesperides, and viewed the still-twitching Ladon.

Ladon was given several parentages, each of which placed him at an archaic level in Greek myth: the offspring of "Ceto, joined in heated passion with Phorcys" or of Typhon, who was himself serpent-like from the waist down, and Echidna or of Gaia herself, or in her Olympian manifestation, Hera: "The Dragon which guarded the golden apples was the brother of the Nemean lion" asserted Ptolemy Hephaestion. In one version, Hercules did not kill Ladon. The creature is associated with the constellation Draco.

The Lightning Thief
Ladon was first mentioned, though not by name, in The Lightning Thief when Luke tells of how the dragon gave him his scar during his quest to retrieve a golden apple from the Tree of the Hesperides. Luke failed the quest but did manage to take one of the dragon's claws that is now in the Big House Attic.

The Titan's Curse
Ladon is first seen in The Titan's Curse when Zoë distracts him while Thalia and Percy sneak by to get to the Mountain of Despair. He is loyal to the Hesperides and is trained to guard the Tree of the Hesperides from anyone. He is trained so well that he does not leave the Tree even when baited. He is partly in cause of Zoë Nightshade's death due to his poisonous bite combined with Atlas's finishing blow. One of his claws (taken by Luke) can be found in the attic of The Big House which holds other spoils of war and the body of the former Oracle.

Trivia

 * According to Hyginus's Astromomy, Ladon is actually the constellation Draco.