Echidna was a monster, half maiden and half serpent, with black eyes, fearful and bloodthirsty. She was the destruction of man, and became by Typhon the mother of the Chimaera, of the many-headed dog Orthus, of the hundred-headed dragon who guarded the apples of the Hesperides, of the Colchian dragon, of the Sphinx, Cerberus, Scylla, Gorgon, the Lernaean Hydra, of the eagle which consumed the liver of Prometheus, and of the Nemean lion. (Theogony of Hesiod 307)
According to Hesiod she lived with Typhon in a cave in the country of the Arimi, whereas the Greeks on the Euxine conceived her to have lived in Scythia. When Heracles, they said, carried away the oxen of Geryones, he also visited the country of the Scythians, which was then still a desert.
Once while he was asleep there, his horses suddenly disappeared, and when he woke and wandered about in search of them, he came into the country of Hylaea. He there found the monster Echidna in a cave. When he asked whether she knew anything about his horses, she answered, that they were in her own possession, but that she would not give them up, unless he would consent to stay with her for a time.
Heracles complied with the request, and became by her the father of Agathyrsus, Gelonus, and Scythes. The last of them became king of the Scythians, according to his father's arrangement, because he was the only one among the three brothers that was able to manage the bow which Heracles had left behind, and to use his father's girdle.