Like the Nukekubi that has their head detach itself when sleeping at night, the Rokurokubi appears like an ordinary woman in the morning but would have her neck stretched and elongated at night.
The neck would stretch so much that it would enable the head to roam freely. It would attack small animals and may just scare people nearby while some story says they would lick up lamp oil with their tongue.
Usually the Rokurokubi doesn’t know that they transform into a yokai at night as it is a curse given to them by some cruel fate. It may or may not be a curse by their own sins. Most likely its not even their own. Rather its either their husbands or fathers’s sins that caused them to be cursed.
The men would commit a grave sin like robbing and killing someone and escaping without punishment thus this sin manifested into a curse as it only effects the female member of the family.
The sad thing about the Rokurokubi is that, they themselves does not know that they become a yokai. Bringing a lot of confusion for the girls on why people are scared of her.