In Japanese folklore, the yokai whose head separates from its body and flies at night is the nukekubi.
A nukekubi may appear human during the day, but after dark its head detaches and travels through the night while the body remains behind. Folklore portrays these flying heads as threatening creatures that may attack people, drink blood, or frighten travelers. Stories sometimes describe a weakness: if the body is moved or hidden, the head may be unable to reunite with it.
The most common mix-up is with the rokurokubi. In many modern accounts, rokurokubi is used as a broad category, but the classic distinction is that a rokurokubi has an unnaturally elongating neck, whereas a nukekubi’s head comes completely off. Japanese traditions also overlap with older East Asian stories about heads that fly away at night.
The motif appears in classical Japanese ghost-story literature, including the 1663 collection Sorori Monogatari. Later retellings, including English-language accounts by Lafcadio Hearn, helped spread both the story and the confusion between the two names.