Although they have never publicly admitted it, her parents were CETO and PHORCYS (or possibly GAIA and Tartarus). She was a strange mix of a serpent with the head of a woman. According to Milton, she ‘seemed a woman to the waist, and fair; but ended foul in many a scaly fold; voluminous and vast.’ But TYPHON obviously fancied her, and she gave birth to most of the classic creatures of horror.
Try not to think NEMEAN-LION, CHIMERA, HYDRA, SPHINX(2), MEDUSA, CERBERUS and ORTHRUS, the Eagle that gnawed PROMETHEUS, Ladon of the HESPERIDES, and various dragons and things. Her children could chew the living daylights out of all but the toughest Heavyweight Champion Deities.
And ZEUS was quite pleased about all this. He enjoyed watching a good fight. He often let the nasties survive to the next round.
ECHIDNA was eventually killed by ARGUS, the hundred-eyed security guard, who only ventured to undertake such a task when she was asleep.