The Rat

The Rat is the true main antagonist from Lady and the Tramp.

Lady and the Tramp
Lady first encounters the rat while she is running happily in the backyard. She sees the rat, growls, and chases after him, causing him to scatter through a hole in the fence. He later has a bigger antagonistic role in the climax when the rat comes back to the house through the hole in the fence while Lady is tied to the dog house at night while it is raining. While Lady barks at the rat, Aunt Sarah tells her to shush. Tramp hears Lady's bark and she tells him that the rat is in the Darling's house. Tramp follows the rat to the baby's room just in time before he can harm the baby. The two engage in a fight, until at one point the rat gets in the baby's crib, about to kill the baby. Tramp chases the rat to the back of a couch, while overturning the baby's crib by accident, causing the infant to cry. Tramp corners the rat on the couch and finally kills him.

Trivia

 * The rat is the only Lady and the Tramp antagonist 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 the baby. However, it is implied as it sneaks into the baby's room, and at one point jumps into the baby's crib, but is thrown out by Tramp.