Nukekubi (ぬけくび) from the Bakemono-Dukushi Yumoto-C
Nukekubi are monsters found in Japanese folklore. By day, nukekubi appear to be normal human beings. By night, however, their heads detach at the neck smoothly from their bodies and fly about independently in search of human prey. These heads attack by screaming (to increase their victims’ fright), then closing in and biting.
While the head is detached, the body of a nukekubi becomes inanimate. In some legends, this serves as one of the creature’s few weaknesses; if a nukekubi’s head cannot locate and reattach to its body by sunrise, the creature dies. Legends often tell of would-be victims foiling the creatures by destroying or hiding their bodies while the heads are elsewhere.
By day, nukekubi often try to blend into human society. They sometimes live in groups, impersonating normal human families. The only way to tell a nukekubi from a normal human being is a line of red symbols around the base of the neck where the head detaches. Even this small detail is easily concealed beneath clothing or jewelry.