Sazae-oni

Sazae oni (Japanese: 栄螺鬼 or さざえおに, meaning "turban snail demon") are monstrous turban snails which haunt the seas in Japanese mythology. They appear on moonlit nights, dancing on the water’s surface like exotic dancers or dragons.

Myths & Legends
Sazae oni are monstrous and deadly creatures, fully deserving the “demon” moniker. They are powerful shape-changers, often taking the form of beautiful women in order to lure seamen into trouble. At sea, they pretend to be drowning victims and cry out to be rescued, only to turn on their would-be saviors once brought aboard. When encountered on land, sazae oni often travel disguised as lone, wandering women who stop at inns and eat the innkeepers during the night.

Attacking Pirates
On the Kii penninsula, legend tells of a band of pirates who spotted a woman drowning in the water one night. They rescued her, though not out of the goodness in their hearts; they had more nefarious reasons to wanting a woman aboard their ship. That night every pirate on the ship had their way with her. Unfortunately for the pirates, the woman was actually a shape-changed sazae oni, and during the night, she visited each pirate on the boat one by one and bit off their testicles. At the end of the night she had all of their testicles, and demanded treasure for their return. The desperate pirates traded away all of their ill-gotten gold to the sazae oni to buy back their “golden balls,” as they are called in Japanese.

Origin
Sazae oni can be born a few different ways. According to ancient lore, when animals reach a certain age, they gain the ability to transform. It was thought that when a turban snail reaches 30 years old, it would turn into a yokai with all kinds of magical powers. Another way that sazae oni come to be is when a lustful young woman is thrown into the sea. Such a woman would transform into a sea snail, and if she happens to live a very long time she will transform into a sazae oni as well.