There are 2 kinds of Rokurokubi:

Rokurokubi whose head comes off (Nukebuki):
The ones whose heads come off has been determined to be the original type of rokurokubi. This type of rokurokubi perform bad deeds such as attacking humans etc. at night, and drinking their blood. For the rokurokubi whose heads come off, there is also the theory that when they sleep (when only the head is flying around), they have the weakness that if the body moves, it can't return to how it was before. In typical classical stories of rokurokubi, when its head comes off at night, someone else would witness it.

-Story:

There is also the theory that for the head to come off is the soul detaching from the body (somnambulism), and in the "Sorori Monogatari", under the title "A Woman's Wild Thoughts Wandering Around" it was interpreted to be a woman's soul detaching from the body during sleep. In the same book, a certain man encountered a nukekubi that changed its appearance into a chick and a woman's head, and when he took out his sword and chased after it, the nukekubi ran away to a home, and from the home, it is said that there was a voice that said, "I saw a scary dream. I was chased by a man with a sword, ran away all the way back home, and woke up."

Rokurokubi whose neck extends:
This type of rokurokubi comes from legends that say that the rokurokubi (nukekubi) have a spiritual string-like object connecting the head to the torso, and it can be said that this type originates from people mistaking the string for an elongated neck.

-Story:

A certain female student was suspected to be a rokurokubi, and when this servant's master went to check on her when she was sleeping, something like steam gradually rose from her chest, and when it became quite thick, her head would disappear, and right before one's eyes, her appearance turned into one with her neck risen up and stretched. Perhaps because she noticed the presence of her surprised master, when the female servant turned over in bed, her neck returned to normal. This female servant was ordinary and other than the fact that she had a pale face, she was no different from an ordinary human, but her master dismissed her. She was always fired wherever she went, and thus had no luck with finding places of employment.
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All I can think of after the last story:

😂