Nukekubi
Nukekubi are monsters found in Japanese folklore and appear to be ordinary people by day but when night falls their heads detach at the neck and fly around in search of human prey.
They are considered a subtype of the Rokurokubi and in general the main difference between the two spirits is that the Nukekubi can detach its head while the Rokurokubi simply extends its neck to impossible lengths.
Legend
According to legend Nukekubi try to blend in with human society and may live in groups, pretending to be human families but can be distinguished from a human by a line of red symbols around the base of their neck symbolizing where their head detaches at night - however they often conceal these marks with clothing or jewelery.
Once the sun goes down Nuekekubi detach their heads from their bodies and hunt down humans, shrieking at their victims before biting them repeatedly - the monsters take a risk in doing this however as their bodies are left immobile in the process and in some legends this serves as the main weakness of these monsters: if a Nukekubi's head is not reattached to its body by sunrise it will die and as such a brave hero or would-be-victim can defeat the monster via destroying its body while its head is off terrorizing.
Popular Culture
The Nukekubi in Hellboy: Sword of Storms.
Nukekubi have appeared in the 1998 Hellboy comic book story entitled "Heads", collected in the Hellboy anthology The Right Hand of Doom. A group of seven Nukekubi appear briefly in the animated film "Hellboy: Sword of Storms", which is partially adapted from the earlier comic story. The Hellboy Nukekubi appeared in action figure form as a San Diego Comicon exclusive, packaged with a Hellboy toy. The toys were designed and released by Mezco.
Even in comic books like Usagi Yojimbo where Usagi confronts a Nukekubi.