This is based off the legend of the Rokurokubi. I was interested in doing this after watching the live action adaptation of Kitaro, which is an older anime. They had one of these characters, a long neck woman, and she was a pretty cool character.
 
Since I’m tired of typing, here’s what wikipedia says:
 
Rokurokubi ( rokurokubi) are ykai 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. They are tricksters by nature, however, and the urge to frighten and spy on human beings is hard to resist. 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.
 
According to some tales, rokurokubi were once normal human beings but were transformed by karma for breaking various precepts of Buddhism. Often, these rokurokubi are truly sinister in nature, 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.
 
Tanuki often imitated rokurokubi when playing practical jokes on people.