Toilet Gods and Demons in Japan
In Japanese folklore there are both toilet gods and toilet demons. The toilet gods are household deities, the toilet demons are water demons. Let's look at the gods first, then the demons.
With only very limited livestock, human waste has traditionally been collected and used as fertilizer in Japan. Cesspits therefore collected the waste, meaning that there was a risk of falling in and possibly drowning. Protection was needed to avoid this dreadful fate. Also, with the collected waste used for fertilizer, the toilet was connected with fertility.
The kawaya no-kami or toilet god was asked for help. Special rituals at the new year asked for the kawaya no-kami to bring a good harvest. Keeping the toilet clean was thought to newborn children more attractive.
The specific name and rituals associated with the toilet god vary with location. The name is Takagamisama around Nagano, Setchinsan in Hiroshima, Kamu-taka on Ishigaki Island, Sechinbisan in Ōita, and Usshimasama in Ehime. In some places the family sit in front of the toilet on a straw mat and eat a rice cake, in other places they would put an offering of food into the toilet.
In far northern Japan the Ainu people believed that their toilet god, the Rukar Kamuy, would be the first deity to provide help in times of danger. The Rukar Kamuy is one of many Kamuy or household gods. There is Apasamn Kamuy the husband and widfe god and goddess who keep evil spirits out of the house entrance. Cisepannokianpa Kamuy and Cisepennokianpa Kamuy, the husband and wife god and goddess of the eaves of the roof, protect the house from earthquakes and typhoons. Cisekor Kamuy, a male god, sits in the east corner of the house and watches over everything. He is the husband of Apehuci Kamuy, the goddess of fire. Colpep Kamuy is the god of containers, thanks are given to this god when containers are retired.
Some branches of Buddhism believe in Ucchuṣma, a god of the latrine who destroys defilement. In Zen monasteries with a belief in Ucchuṣma, the latrine, the bath, and the meditation hall are the three sanmokudō or "silent places" for contemplation.
Japanese folklore describes many yōkai, or demons or imps. Many of these are associated with water. Shintō has many Suijin or 水神, or water deities. Japanese Buddhism describes a type of hungry ogre.
The one we're interested in is the Kappa or 河童, the River Child.
The precise description of the Kappa varies from location to location, but the common features are a shell and beak similar to a turtle plus the sura or plate, a dish-like area holding water on top of the head. The sura always holds water and is the Kappa's source of power. If the sura is emptied while away from water, the Kappa loses its power and may die.
Below is a depiction of twelves forms of Kappa.
The Kappa goes by many names. It's also known as the Kawatorō or 川太郎, or the Komahiki or 駒引, or the Kawako or 川虎, literally river tiger. Regions have their own names, with Kappa being a generic term for what are known by dozens of local names. An equivalent is known in Chinese folklore as Shui Gui or 水鬼, "Water Ghost".
The Kappa is a trickster figure. Some of what they are said to do is merely mischievous, such as looking up women's kimonos. But they are credited with drowning people and animals (especially horses and cattle), raping women, and kidnapping children. They are said to sometimes drink blood and eat human flesh, especially the liver.
That leads to their toilet-related lore: The Kappa is said to climb up inside the anus of a person using the toilet in order to eat or steal their liver or the shirikodama, a ball inside the anus said to contain the soul.
All is not lost if you encounter a Kappa, as long as you remember that they are obsessed with politeness. Bow to the Kappa and it will be obligated to return the gesture. That spills the water out of its sura on top of its head, rendering it unable to move. Refill the sura, and the Kappa will serve you forever.
Another thing to try would be to bribe the Kappa with food — eggplant, soba buckwheat noodles, or its favorite, cucumbers. People sometimes write their names, or their children's names, onto cucumbers and toss them into water believed to be infested with Kappas in order to safely swim or bathe. In some regions, eating cucumbers is thought to be a defense against the Kappa, but in others it is thought to be suicidal, turning yourself into what a Kappa would see as a tasty cucumber-filled treat. Related to this, the kappamaki is a sushi roll filled with cucumber.
Once you have obligated a Kappa into being your life-long assistant, they can be very helpful with water-related tasks.
They can bring fish, and help farmers to irrigate their fields.
They are also believed to be very knowledable about human medicine, and to have provided the first explaination of how to set broken bones.