Nasty Jack, the meanest horse thief, is the leader of a vicious group of horse thieves (i.e. horses who steal things). According to one song line from the episode, Jack is a thoroughbred and cannot be outrun. He considers himself to be the "nastiest horse thief." Part of what makes Jack so feared and cruel is what he does to sheriffs, whom he shows great hatred toward. Using his strength, he "trounces" the sheriff, meaning that he takes them, puts them in a bag, and simply drops them. When Piglet reluctantly and unexpectedly becomes the new sheriff of the town, he becomes Jack's newest target.
Role in the series
The episode's framing device focuses on a play being performed by Winnie the Pooh and friends, which is titled "The Legend of Sheriff Piglet." Jack features as the main villain in the play. He and his gang come to the town of Rickety Gulch to terrorize the town, where they reside in a saloon.
When Piglet reluctantly and unexpectedly becomes the new sheriff of the town (as Piglet now possesses the sheriff badge), Jack makes plans to "trounce" Piglet. The group is able to escape into the sheriff's office. Jack later intimidates Rabbit into serving his gang banana splits. Rabbit does a poor job of making the splits and is tormented by Jack for it. Tigger attempts a rescue, but Jack just laughs as Tigger gets tangled up in his own lasso. Jack prepares to trounce Piglet but is confronted by the Masked Bear (Pooh's alter ego). The two engage in a duel using ice cream scoops (and containers of ice cream) as weapons. Jack corners the Masked Bear, but a stray scoop of ice cream results in the head horse thief being sent flying into a water trough.
Later, Jack sets a trap for the Masked Bear and captures him and Eeyore his faithful steed). He then challenges Piglet to a showdown. Piglet is able to gather the courage to face Jack; however, in a twist of fate, his badge falls off. As Piglet is no longer Sheriff, Jack can no longer trounce him. At Piglet's suggestion, Jack decides to become good, and becomes Sheriff himself, stating that he always wanted to be Sheriff. At the sight of this, the rest of the horse gang flee town after their once-villainous leader promises to clean up the town. Jack's final appearance is at the ceremony where Piglet is honored as a hero.
It is possible that the reason Nasty Jack hates sheriffs and became a horse thief in the first place was because he either never had the chance of becoming Sheriff himself, or nobody ever made him Sheriff at all until his run-in with Piglet, since he stated that he "always wanted to be Sheriff," thus reforming.