“When a mouse or a rat reaches one thousand years of age, it turns into a gigantic rodent yōkai [spirit] called a kyūso [cornered rat]. They look like ordinary rats, only they are as large as cats or even medium-sized dogs.
“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 instead hunt 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.
“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.”
– “Cornered Rat (Kyuso)”, Yokai Wiki