Ladon

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

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, who successfully stole one of these apples. The following day, the Argonauts, who were on their return journey from Colchis, passed by and heard the lament of "shining" Aigle, one of the Hesperides, and viewed the still-twitching Ladon.

Ladon was given several different parents depending on the myth, 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 Gaea 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. Also, in some stories, the dragon was said to have a hundred heads.

The Lightning Thief
Ladon is mentioned, though not by name, 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 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 the cause of Zoë Nightshade's death due to his poisonous bite combined with Atlas' finishing blow. One of his claws, being 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.

The Bronze Dragon
Ladon is referred to when Percy says that the latter had faced dragons with so many heads that "if you stopped to count them you'd be pretty much dead".

Trivia

 * According to Hyginus' Astronomy, Ladon is actually the constellation Draco.
 * Ladon was the one that gave Luke Castellan the scar on his face.