Kyuso is a specter in Japan which has the form of a mouse and is said to have lived for hundreds and thousands of years by undergoing transformation.
It is a mouse about the size of a medium-sized dog and is said to eat even a cat; on the other hand, sometimes it is said to mate with a cat and rear its blood-related kittens (as later described). Some are said to have lived for over three thousand years.
According to "Ehon Hyaku Monogatari" (Picture Book of a Hundred Stories), a collection of strange stories during the Edo period, kyuso lived in a stable of a certain house during the Bunmei era. This bogy turned into a specter over the years; it became an extremely giant mouse which was said to have lived over three thousand years. One day, a cat appeared under the above-described circumstances, mated with kyuso, and bore five kittens. However, the mother cat later ate poison and died. Kyuso saw the motherless kittens every night and took care of thm; then once the kittens were safely grown up, kyuso disappeared somewhere.
According to the same book, kyuso in Yamato Province (present-day Nara Prefecture) was a calico cat with the hair of red, white and black colors, and was said to be always eating cats.
Furthermore, the following description is contained in "Okinagusa", an essay written during the mid-Edo period.
This is a story taking place in around 1751 in Owari Province, Nagoya-go (present-day Nagoya City, Aichi Prefecture) in Aichi-gun (Aichi Prefecture). At a certain house, andon (paper-covered lamp stand) was burning out almost every night; when the cause was searched, it was discovered that kyuso had been appearing in the middle of the night to lick the fish oil that was used for the light.
A cat was prepared in hope of getting rid of the bogy mouse; then kyuso appeared.
The cat jumped at kyuso menacingly. However, the cat failed to catch kyuso and instead, kyuso bit into the cat's throat, killed it, and shrewdly ran away.
People in the household were very much surprised, and after poking around various places in search of a cat which was good at killing mice, once again challenged kyuso. The night came, and finally kyuso and the cat got ready for a fight.
Time ticked away, and it is said that eventually kyuso moved and the two fought; once again, however, kyuso bit the cat to death and ran away.
As such, people observed a scene just like the one described in the proverb, "a cornered mouse will bite the cat", running true to form.