Echidna

The Echidna (Έχιδνα in Ancient Greek, meaning "she viper") is considered the mother of all monsters. Typhon is her husband. Percy, Annabeth, and Grover encounter her in The Lightning Thief on the top of the St. Louis Arch. Her children include:


 * Nemean lion
 * Ladon
 * Chimera
 * Sphinx
 * Hydra
 * Cerberus
 * Orthrus
 * Ethon
 * Teumessian fox
 * Crommyonian sow
 * Colchian dragon

History
Echidna was usually considered an offspring of Tartarus and Gaia, or of Ceto and Phorcys (according to Hesiod) or of Chrysaor and the naiad Callirhoe, or Peiras and Styx (according to Pausanias, who did not know who Peiras was aside from her father). Echidna was a dracaenae, with the face and torso of a beautiful woman (depicted as winged in archaic vase-paintings) and the body of a serpent, sometimes having two serpent's tails. She is also sometimes described as Karl Kerenyi noted an archaic vase-painting with a pair of echidnas performing sacred rites in a vineyard, while on the opposite side of the vessel, goats were attacking the vines: thus chthonic Echidnae are presented as protectors of the vineyard.

The site of her cave Homer calls "Arima, couch of Typhon". When she and her mate attacked the Olympians, Zeus beat them back and punished Typhon by sealing him under Mount Etna. However, Zeus allowed Echidna and her children to live as a challenge to future heroes. She was an immortal and ageless nymph to Hesiod (Theogony above), but was killed where she slept by Argus Panoptes, the hundred-eyed giant.

Series
In The Lightning Thief, while Percy, Annabeth, and Grover are at the top of the arch they encounter Echidna who also has the Chimera with her. She quickly isolates Percy from his friends and pits them against her monstrous son at the top of the arch. Percy barely manages to escape after being poisoned by the Chimera's snake tail and plummeting into the water below.

Physical description
In the book Echidna is described through the mist as an extremely fat woman with a chihuahua. The only clues that she was a monster were her reptillian tongue and the fact that she called her dog 'Sonny'.

Noteworthy
While Echidna orchestrates the fight between Percy and the Chimera it should be noted that she herself does not go into direct combat with him and only appears in the first book. She, along with Chimera, are a few of the only monsters that appear in the series that are not defeated and sent to Tartarus. This leads to questions about an uninterrupted fight between Echidna and Percy.

Trivia

 * In The Lightning Thief, the part where Percy and his friends encounter Chimera and Echidna was replaced with them fighting the Hydra at a replica of the Parthenon in Tennessee.