Rokurokubi mostly look like humans with the sole difference being that at night they have extremely long, flexible necks. In fact, they look so similar to humans that they are sometimes portrayed as not being yōkai. Instead, they are sometimes portrayed as ordinary humans who are under a curse or who have a medical condition. They are usually, but not always, female.

These yōkai are not very dangerous. They are mostly just known for eating small animals and licking lamp oil, although they do sometimes cause mischief by scaring nearby people. This often does more harm to the yōkai than the people it scares as, according to at least one legend, rokurokubi have been known to lose their jobs once their identity is discovered. This makes it difficult for them to stay employed for long.

FUN FACT: This yōkai is closely related to nukekubi, a yōkai that can detach its head from its neck. Nukekubi are the more dangerous of the two as their heads are known for flying around in search of human flesh and blood.

Examples in various media:

Miki (Hell Teacher Nube)

Lady Longnek (Yo-Kai Watch)

Unnamed Rokurokubi (Hellboy Animated: Sword of Storms) 

Rokurokubi are also depicted in the Hellboy comics.