From the Super Mario Wiki, the Mario encyclopedia
The Sheep Rustlers are a trio of characters appearing in the Saturday Supercade episode Sheep Rustle Hustle. They are despised for persistently stealing and smuggling sheep, and will go to great heights to do so, even if it means capturing those who attempt to meddle with them.
They first appeared at Biff's ranch to steal his sheep, along with King when he tried to stop them. Biff gave chase, and was assisted by Donkey Kong Jr. and Bones. Bones attempted to fall into their cargo truck, but was spotted. They also captured Biff when he and Donkey Kong Jr. tried to assist, though Bones was freed in the process.
While they were mulling over how much money they would make from the sheep, Donkey Kong Jr. and Bones managed to trick two of them into entering the saloon, where they were haunted. This strategy, however, also failed, and though Donkey Kong Jr. was able to steal the jail key, they were caught by the third member, who locked them up.
The group then loaded the sheep onto a cargo train to escape; however, Donkey Kong Jr. managed to lasso the train's smokestack and use the water from a water tower to wash out the Rustlers before they could get too far, while Bones and Biff tied them up. It is unknown what happened to the Rustlers after that, though presumably they were locked up in the same jail where they locked up the protagonists earlier.