sankai, literally, "birth monster") is a generic term that refers to any Japanese regional yōkai (praternatural creatures) that is given birth by a pregnant woman. When due care is not given during pregnancy, it is said that a sankai would emerge instead. Before the advent of modern medicine, childbirth was far more dangerous and incomprehensible, and people were deeply superstitious. Thus, unusual appearances from miscarriages and early births created the sankai belief.
The story of the oketsu (オケツ) comes from Okayama Prefecture. From outer appearance, it looks similar to a turtle, and it has hair growing on its back. As soon as it is born, it starts to crawl on the floor and attempts to escape underneath the house. If not captured and killed right away, it is said to crawl underneath the sleeping mother and kill her.
KekkaiThie sankai is known as a legend about its outer appearance is sparse, however it is said to look like cattle and is said to be hairy. Kekkai is also said to bury under the house to kill its mother.A practice existed of surrounding the bottom of a house with byōbu during childbirth to prevent this maneuver. it is theorized that the etymology may derive from kekkai (結界), meaning a restricted area for religious reasons.
Folklorist Iwao Hino recalls spotting a kekkai at a freak show during childhood. The show stated that it was given birth by a woman at a University hospital, however Hino later believed that it must have been a trained night monkey.