Charybdis

Naiad daughter of Poseidon, she stirred things up by steering the tides where water was apt to rise, claiming for Poseidon more ocean-front ocean and pissing Zeus the hell off.

So he turned her into a monster.

Not just any monster, either, but a seawater-sucking whirlpool of a monster. That's her, near the bluff of Scylla. Woe betide the sailor who sails between the rock and her hard place.