Medusa
 Medusa, daughter of Phorcys and his sister Ceto, sister of Stheno and Euryale, was alone mortal among her sisters. She was slain by Theseus and her blood spawned Pegasus. She is guarded by the Three Old Hags, Buzzer, Terror, and Fury (I couldn't find a place to link them to), who among the three of them have one eye and one tooth. Medusa is depicted as slender and evil, with snakes for hair and a look which literally turns any man who gazes into her eyes into stone. She was originally beautiful, but after she slept with Poseidon in Athena's temple, Athena turned her into a hideous monster with snakes for hair.
In the lightning thief, Medusa is depicted as Aunty Em, an old lady who sells stone garden statues, which are the people she turned to stone. Percy slays her with Riptide, looking at her reflection in a crystal ball.
This is an image of Theseus slaying Medusa (only classic image available).

external image MedusaS.jpg

No author available. Wikipedia. No editor available. September 19, 2010.
Wikipedia. September 20, 2010. <http://en.wikipedia.org/wiki/Medusa>
Rick Riordan. The Lightning Thief. New York: Hyperion books, 2006.
W.H.D. Rouse. Gods, Heroes and Men of Ancient Greece. New York: Mentor, 1957.