A Sumo Wrestler's Tail is an adaption of a folktale called Nezumi no Sumō.
(translated from Museum website) Once upon a time, there was an old man and his wife who lived on a mountain. One night, the old man found a bunch of mice heading off to somewhere. He followed them and saw the mice holding a sumo tournament. The man and his wife happened to know that the mice living in their house always lose in these matches, and so the old couple give the mice some food to support the team.
An old farmer discovers a group of rats heading to a sumo wrestling bout. After they lose miserably, he decides to feed the rats to boost their chances of winning.
- A Sumo Wrestler's Tail on Anime News Network
- A Sumo Wrestler's Tail on IMDb
- A Sumo Wrestler's Tail on Wikipedia