Echidna is a Greek monster depicted as being half snake, half woman in Greek mythology. She is also known as the "mother of all monsters," as she is mother of a large number of the monsters and mythical creatures such as the Chimera, Cerberus, Orthrus, the Lernaean Hydra, the Sphinx and the Nemean Lion, among others. She was the wife of Typhon, who was the "father of all monsters" and the most fearsome and dangerous monster in said mythology.
Her parentage is debatable, but it is most often stated that she was the daughter of either Gaia and Tartarus, or of the sea monsters Phorcys and Ceto. She was eventually slain by Hera's servant, the hundred-eyed giant, Argus.
"Origin: Greece. The mother of all monsters. Her upper half is a beautiful woman, and her lower half is a spotted snake. Her offspring include such beasts as Orthrus, Cerberus, Hydra, Chimera, and Ladon."
—Shin Megami Tensei A-Mode DDS dictionary
In the GBA/iOS versions, if Pascal-Cerberus's mind is restored with the Golden Apple at the entrance to Tokyo Destinyland, the protagonist will receive the Dog Collar Visionary Item upon Echidna's defeat.
Majin Tensei II: Spiral Nemesis
Echidna is a boss in Paranoia's Elleth on the Dark-Law/Chaos routes.