Oceanus' daughter Callirrhoë. (Other accounts give her parents as Tartarus and Gaia, however, or Ceto and Phorcys.) She was part beautiful woman, part snake, and bore Chimaera, Hydra and Cerberus (a fearsome threesome!) to Typhon. She also bore the Sphinx by her brother Orthus. Her name simply means 'viper', 'adder' (echidna).
By something of a semantic twist, the modern English word 'echidna' is used for the creature known as the spiny ant-eater, otherwise, as Chambers Twentieth Century Dictionary describes it, 'a genus of Australian toothless, spiny, egg-laying, burrowing monotremes'. This sounds even more horrific than the original mythological monster.