Scylla

Today's Greek mythological beastie is Scylla. You may know her from the Police song "Wrapped Around Your Finger". 
She was on one side of a narrow strait of water, and Charybdis, a creature that would suck down whole ships in a whirlpool in order to eat the crew, was on the other side.
So, just like being caught between a rock and a hard place, sailors would have to choose whether to lose six of their crew, Scylla had six dog heads that would snatch the men from the deck of the ship, or risk losing everything to the whirlpool.
The stories vary about how this beautiful woman came to be transformed into this monster, but it is safe to say that it was tough to be good looking back then.
Goddesses and other magical beings were very jealous in those days, and often put the smack down on those blessed with straight teeth and clear skin.
Things must have loosened up quite a bit in our times, otherwise Hollywood would be full of monsters.