In the pages of The Dark Knight Returns, the Mutants are a youth gang that was formed sometime after Bruce Wayne retired as Batman.
When the Mutants cause trouble in Gotham City at the time when crime is at an all-time high, Bruce Wayne becomes Batman again.
During their hand-to-hand brawl, Batman, despite being able to match the Leader in strength, is rusty and slightly slow due to a decade's lack of activity which results in him getting seriously injured.
[1] The mainstream version of the Mutants were seen causing trouble until they fight Dick Grayson as Batman and Damian Wayne as Robin.
[3] When Booster Gold accidentally brought Jonah Hex to the present, he finds a family being endangered by the Mutants led by King-Boss-Champion Steaktube.