In Greek Mythology how did Echidna become a monster?

The Greek goddess Gaea was goddess of Earth and mother of many kinds of beings, from the Titan gods to the Giants, to immortal beings that could be called monsters.

Echidna was one such immortal "monster", a immortal who presided over rot, slime, fetid waters, illness and disease. She gave birth to other immortal "monsters" or beasts by the immortal storm giant Typhoeus who challenged Zeus in heaven. Her children were said to be the Chimaera, the many-headed dog Orthus, the hundred-headed dragon who guarded the Apples of the Hesperides, the Colchian dragon, the Sphinx, Cerberus, Scylla, Gorgon, the Lernaean Hydra, the eagle which consumed the liver of Prometheus, and of the Nemean lion.

Her mother is sometimes Gaea and her father Tartarus; or the ancient sea gods Phorcys and Keto (also children of Gaea).