the online database of Japanese folkore



Translation: former rat, old rat
Alternate names: yōso (strange rat)
Habitat: barns, houses, fields
Diet: cats, and pretty much anything else it wants

Appearance: When a mouse or a rat reaches one thousand years of age, it turns into a gigantic rodent yōkai called a kyūso. They look like ordinary rats, only they are as large as cats or even medium-sized dogs.

Behavior: In addition to growing larger and stronger than regular rats and mice, kyūso begin to exhibit peculiar behaviors. Aware of their own size and strength, they no longer scurry away at the first sign of danger. Instead of being chased by cats, they sometimes hunt them down and eat them, reversing their historical relationship.

Despite their ferocity, kyūso are not entirely monstrous. There are a number of folktales and even modern urban legends about kyūso playing with cats, or even raising litters of abandoned kittens as if they were their own children.

Interactions: Even more disturbing than their strange relationship with cats, kyūso are said to occasionally sneak into homes at night and have sexual relations with young women.

Origin: Kyūso is famously depicted in the Ehon hyakumonogatari and a number of other Edo Period sources, however stories about giant yōkai rats have been a part of Japanese and Chinese folklore since ancient times. Its name is likely a pun. There is an old saying: “kyūso neko wo kamu” (“a cornered rat will bite a cat”). Kyūso (“cornered rat”) and kyūso (“old rat”) are homophones, and it’s possible that this yōkai was invented based on this wordplay.

Legends: A tale from 15th century Dewa Province is recorded in Ehon hyakumonogatari. It tells of a kyūso which had taken up residence in a family’s barn. The family also kept a female cat. She and the kyūso came to be very good friends. One day, the cat became pregnant. She bore a litter of five kittens, but tragically died shortly after giving birth. In her place, the kyūso visited the kittens every night and took care of them. When they had all grown into adult cats, the kyūso vanished from the barn, never to return.

In Nagoya in the 1750’s, a family was perplexed as to why their lamps were extinguishing themselves every night. Finally they discovered the cause: every night a gigantic rat would appear and lick up all of the oil from each of the house’s lanterns. In order to stop the rat, the family bought a cat. They waited until nightfall and released the cat near where they saw the kyūso. Sure enough, it appeared. The cat leaped upon the kyūso, but the rat was stronger. It tore open the cat’s throat and escaped.

The family was shocked. They searched all around town until they found a big cat who was very skilled at catching rats. They purchased the cat, and once again released it into the house. That night, when the kyūso appeared, the two creatures locked eyes and began to snarl at one another. Finally, the kyūso couldn’t hold back any more. It leaped at the cat, and tore its throat open just as it had done so before.

A folktale from Kagawa Prefecture describes one instance where a kyūso finally loses to a cat. The cat was a stray taken in by a priest named Ingen from Ōtaki Temple. A huge rat weighing over 26 kilograms was living in the temple’s main building, and had been terrorizing the priests for years. The cat was too small to kill the kyūso by itself. However, after three years of living there, the cat’s tail split in two and it became a cunning nekomata. Out of thanks for Ingen’s kind treatment, the nekomata enlisted an army of local cats to drive the kyūso out of the temple. After a long and bloody battle, the cats were finally able to kill the kyūso.

Alphabetical list of yōkai