Rokurokubi are demons found in Japanese folklore. They look like normal human beings by day, but at night they gain the ability to stretch their necks to great lengths. They can also change their faces to those of terrifying oni to better scare mortals. In their daytime human forms, rokurokubi often live undetected and may even take mortal spouses. Many rokurokubi become so accustomed to such a life that they take great pains to keep their demonic forms secret.
Whirlpool and Waves at Naruto, Awa Province
Whirlpool and Waves at Naruto, Awa Province, Ando Hiroshige
Carp in a Pond, by Kasamatsu Shirō, 1957
14 Terrifying Japanese Monsters, Myths and Spirits
The Tale of Koheiji by Hokusai. It was based on an actual event, Koheiji was killed by his wife and her lover. As revenge he returns to haunt the couple while they are in bed together inside mosquito netting.