Monsters: Yama-Uba

Yama-Uba (Mountain Hag) is a female yokai who are considered old hags or witches in the Japanese Forests and Mountains. The Yama-Uba were born human but turn into monsters. They typically appear as kind old ladies, but sometimes they sport horn or fangs. They don't give sign to their evil nature.

Yama-Uba will live alone in huts by the road. They will offer shelter, food, and a place to sleep for travlers. Unbeknowist to the travlers she would wait for them to sleep before turning to their true form an old, ugly, demonic witch. She would then catch and feast on them using her magic. 

Stories of the Yama-Uba would spread and keep travlers to be safer. It's also told as bedisde stories to disobediant children. Sometimes these stories were told when young wome were accused of crimes or wicked deeds and they had to flee to the forest/mountain. They would live in exile becomeing an "old hag".

Sometimes during famines or economic hardship families would make hard choices to get rid a family member so the rest would survive. Often this was a newborn or an elderly family member would be removed. It's believed that some of the Yama-Uba may have been elderly senile women abandoned by family members in the forest or mountains. It's said that these women would either out of rage or desperation tranformaed into the Yama-Uba practicing black magic. 

SOURCES:

Yokai Wiki

Wikipedia

Yokai