The Rokurokubi is an Edo-period creature that lives as an ordinary person by day but whose neck stretches out long as a serpent at night. Some tales hold that the head wanders without the woman herself even knowing.
Its core is the horror of mimicry — the dread that a near neighbor or family member may not really be human. Because the everyday body grotesquely transforms by night, it reads as an archetype of body horror.