Cujo the St. Bernard

Cujo is a St. Bernard dog and the central antagonist of the film of the same name.

About
Cujo is a regular dog for the first part of the film, but a bite from a vicious bat turns the once friendly St. Bernard into a terrible rabid beast that kills anyone who is daring enough to cross paths with him. Donna Trenton and her son, Tad, are going to Cujo's owner's house when their car breaks down. To make things worse, Cujo, who had already claimed two victims in the house, is lurking just outside the car, ready to attack them. He attempts to kill Donna and Tad continuously while they are stuck in the car. Donna tries several times to repair the car, but Cujo attacks her. The two of them run into the house. While inside, Tad faints of thirst. Donna finds a baseball bat and hits the dog with it repeatedly. When the bat breaks, she stabs him with it. The dog drops, thought to be dead. Donna runs to the kitchen where the Cujo who was "playing dead", leaps through the window and attempts to kill her.

Nobody can stop this beast until Donna kills him by shooting him with the pistol that belonged to the Sheriff that Cujo killed, murdered, and slaughtered earlier.

Trivia

 * Reggie from Disney's Lady and the Tramp II: Scamps Adventure is based on Cujo because Reggie from Lady and the Tramp II: Scamps Adventure does have the same bad violent behavior as Cujo.
 * Since it was a bat that bit Cujo and infected him, the bats are often considered the true main antagonists.