Cujo

Cujo is the titular main antagonist of Stephen King's 1981 novel and its 1983 film adaptation of the same name, and a minor antagonist in Cat's Eye. He was once a friendly and playful companion, but when he got rabies, he went insane and was turned into a killer that attacked anybody who came near.

He was portrayed by Daddy and his vocal effects were provided by Frank Welker. Cujo is given some more insight in the novel while the film portrays him as merely a dog gone mad due to rabies. It is said in both versions that Cujo was friendly dog that loved to play with the local children. After being bitten by a bat and unknowingly infected by the virus, he was afraid his master would be mad and think he was a bad dog showing how much he thought of his owners and their opinions. Cujo, when infected, starts to slowly lose his mind to where he could no longer recognize even his owners (the young Brett Camber) as human seeing them akin to actual monsters barely recognizing his voice and fights off the urge to kill the boy by leaving.

Based on Cujo's perspective, he seemingly only identifies them through small descriptions in all caps like: "THE BOY" and "THE MAN" when thinking of them. As the book progresses, we see that he does not kill for pleasure but due to some of their loud noises and other habits which intensifies his pain thus making him lose what little restraint he manages to retain. Cujo wants to be left alone after being infected to the point he is willing to kill those around him if needed. As his mind continues to deteriorate, Cujo becomes more violent and insane from the pain and starts to believe it is the people around him causing him to be sick. All in all Cujo is not a creature that seeks to harm others in fact the book ends making it clear that Cujo himself only ever wanted to be a good dog who after his infection it drove him crazy and was no longer in full control of his actions.