Echidna was an immortal goddess who was part beautiful woman and part fearsome snake. She is said to have given birth to most of the monsters in Greek mythology, earning her the appropriate nickname "Mother of All Monsters".
There seems to be some question as to her own lineage, as some Greek writers think Tartarus and Gaia raised her, and others think she was the offspring of Peiras and Styx. Regardless, after she and Typhon attacked the Olympians, Zeus mercifully spared her life, deciding to instead leave her as a challenge for future heroes - that is, until she was killed in her sleep.
Image credit - Sarapsys