A Futakuchi-onna (二口女?, lit. "two-mouthed woman") is a type of yōkai or Japanese
monster. They are characterized by their two mouths – a normal one located on her face and second one on the back of the head beneath the hair. There, the woman's skull splits apart, forming lips, teeth and a tongue, creating an entirely functional second mouth.
In Japanese mythology and folklore, the futakuchi-onna belongs to the same class of stories as the rokurokubi, kuchisake-onna and the yama-uba, women afflicted with a curse or supernatural disease that transforms them into yōkai. The supernatural nature of the women in these stories is usually concealed until the last minute, when the true self is revealed.
The origin of a futakuchi-onna's second mouth is often linked to how little a woman eats. In many stories, the soon-to-be futakuchi-onna is a wife of a miser and rarely eats. To counteract this, a second mouth mysteriously appears on the back of the woman's head. The second mouth often mumbles spiteful and threatening things to the woman and demands food. If it is not fed, it can screech obscenely and cause the woman tremendous pain. Eventually, the woman's hair begins to move like a pair of serpents, allowing the mouth to help itself to the woman's meals. While no food passes through her normal lips, the mouth in the back of her head consumes twice what the other one would. In another story, the extra mouth is formed when a stingy woman is accidentally hit in the head by her husband's axe while he is chopping wood, and the wound never heals. Other stories have the woman as a mother who lets her stepchild die of starvation while keeping her own offspring well fed; presumably, the spirit of the neglected child lodges itself in the stepmother's or the surviving daughter's body to exact revenge.
In Popular Culture
In Pokemon, there is a Pokemon from Generation 3 known as Mawile based on this cryptid. It is a Steel/Fairy type. It lures its victims with its docile-looking face, then it bites them with its large jaws. The jaws are steel horns that got transformed. They also can't taste anything, making it the mouth Mawile uses to eat food it does not enjoy.