Gazu Hyakki Yagyō
Teeth blackening was performed in medieval Japan among males of the nobility (kuge) and warrior class (buke), so it is inferred that they may be a yōkai of someone from this class, or alternatively they may come from how monsters in Omine, Aso District, Kumamoto Prefecture are called "wanwan" and how monsters in Taniyama, Kagoshima Prefecture (now Kagoshima) are called "wan". In the background of the picture in the "Gazu Hyakki Yagyō", there is the interpretation that they are a paranormal phenomenon that appears from the walls of a deserted residence.