Charybdis

Charybdis was a Greek sea monster of terrifying proportions.

History
Charybdis was a nymph born out of union of Poseidon  and Gaia. When her father had quarrel with Zeus, Charybdis supported Poseidon to an extent that didn't please Zeus. As Charybdis had been tasked with flooding dry lands, the king of the gods cursed her to become a gargantuan monster to live beneath a rock at the Strait of Messina. There she would forever swallow water three times a day, creating periodical whirlpools that threatened to sink entire ships.

Other side of the strait was not safe either, as another monster, Scylla, lived there. Anyone wishing to pass through had to decide if it was worth it to risk losing whole ship, or to lose some of the crew for certain.

Appearance
After Zeus had curse Charybdis, she became an enormous monster that mostly consisted of a bladder, with flippers and fins as arms and legs.