The Rat

The Rat is a vicious, nameless rodent, and the main antagonist (replacing Aunt Sarah in the climax) of Disney's Lady and the Tramp. It also made numerous appearances in several other productions outside the original film.

Lady and the Tramp
Lady first encounters the rat while she is running happily in the backyard. She sees it, growls, and chases after it, causing it to scatter through a hole in the fence.

The rat has a bigger antagonistic role in the climax of the film, when it comes back to Jim Dear and Darling's house through the fence hole while Lady is chained to her doghouse on a stormy night. While she barks at it, Aunt Sarah tells her to shush. Tramp hears her barks, and she tells him that it is in the house. He follows it to Jim Jr.'s room just in time before it can harm him. They engage in a fight, until at one point, it gets to the top of Jim Jr.'s crib, about to kill him. Tramp chases the rat off, overturning the crib by accident, and causing Jim Jr. to cry. Finally, he manages to corner it behind the couch and kill it.

Having been alerted by the commotion, Aunt Sarah arrives and, not seeing the rat, mistakenly believes that Lady and Tramp had attempted to harm Jim Jr., promptly locking Lady in the cellar and sending Tramp off to the pound. However, as soon as Jim Dear and Darling return, Lady shows them the dead rat, clearing Tramp's name.

Trivia

 * In an early script, the Rat's name was Herman.
 * The Rat is the only antagonist in the film that doesn't make any sound whatsoever other than a few squeaks, while it was being attacked by Tramp.
 * It is never outright stated that the Rat wants to harm Jim Jr.. However, it is implied as it sneaks into his room and, at one point, jumps into his crib, but is thrown out by Tramp.
 * The Rat was the only villain to die at the end.
 * The Rat's gender is unclear, perhaps male since it was named Herman in the early script.
 * The Rat was animated by Wolfgang Reitherman, who also animated Monstro, the climax's antagonist of Pinocchio.
 * The Rat's breed might be a Brown Rat.
 * The Rat is the main antagonist of the film because, despite being a creature, it had bigger plans than anyone else.