Rokurokubi are yokai found in Japanese folklore.
According to some tales, rokurokubi were once normal human beings but were transformed by karma for breaking various precepts of Buddhism.
Rokurokubi look like normal human beings by day, but at night they gain the ability to stretch their necks to great lengths. The head can be seen floating independentlty from the sleeping body, perpetrating mischief as sucking the life energy out of people and animals, and licking up the oil of andon lamps. They can also change their faces to those of terrifying oni (Japanese ogres) to better scare mortals. Tanuki often imitated rokurokubi when playing practical jokes on people.
Due to an error made by Lafcadio Hearn in his book Kwaidan: Stories and Studies of Strange Things, these creatures are often mistaken for a more vicious monster, the nukekubi.
Most rokurokubi are truly sinister in nature, going as far as eating people or drinking their blood rather than merely frightening them. These demonic rokurokubi often have a favored prey, such as others who have broken Buddhist doctrine or human men. Some rokurokubi thus resort to revealing themselves only to drunkards, fools, the sleeping, or the blind in order to satisfy these urges. Other rokurokubi have no such compunctions and go about frightening mortals with abandon. A few, it is said, are not even aware of their true nature and consider themselves normal humans. This last group stretch their necks out while asleep in an involuntary action; upon waking up in the morning, they find they have weird dreams regarding seeing their surroundings in unnatural angles.
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.