He was a St. Bernard dog male owned by the Camber Family. Cujo was once a friendly and playful companion, but when he contracts rabies from a bat bite, he went insane and was turned into a vicious killer that attacked anybody who came near him. Cujo can be seen as a tragic being as he actually doesn't want to kill anyone, but the disease had taken over his mind and turned him into a feral beast.
During the events of the novel, Cujo was bitten by a bat and got rabies from it. For the next few days, he became very bloodthirsty and dangerous, eventually killing Gary Pervier and his owner, Joe Camber. When Donna Trenton and her son, Tad, go to the Camber's farm for an auto repair, they are attacked by Cujo and spend three days in their car hiding from him. By this time, Cujo is now completely covered in blood, grime, froth and mucus, and the smallest things could set him off. When the phone rings, Cujo flies into a rage and viciously attacks the car, breaking a window in the process. Cujo bites Donna in the leg when she tries to escape and get help and she barely survives the merciless and destructive attack. Luckily, the sheriff arrives, only to be savagely mutilated by Cujo after a brief struggle.
Meanwhile, Tad is dying of severe dehydration and Donna decides that she has to do something or else her son will die. She makes a daring escape from the car and Cujo attacks. Luckily, Donna is able to grab a discarded baseball bat from a junk pile and fend the dog off. She beats the dog with the blunt instrument, but it breaks when it strikes Cujo's face. Cujo lunges, and she falls backwards. However, when the dog jumps on her, she jams the broken end into Cujo's stomach, impaling him. Donna is traumatized and throws the apparently dead dog off of her and uses the sheriff's revolver to break her son out of the car. She takes him inside and gives him water. As he starts to revive, a barely alive Cujo uses the last ounce of his strength to jump through a window. The mortally wounded Cujo lunges, but Donna manages to retrieve the sheriff's revolver from the counter and shoot him in the head, killing him.
In the book, Cujo's death is much more graphic. The end showdown is the same, but when Donna breaks the bat, she impales him in the eye with the sharp broken end, killing him instantly. A local county health official arrives and cuts off the dog's head to send to the state Department of Health and Welfare.
Other Appearances
Along with the possessed automobile Christine he makes a cameo appearance in "Cat's Eye" where he chases the titular cat General. He kills with multiple bite and scratch wounds.
In an episode of Will & Grace, Karen mentions Cujo's name when Will's dog begins licking the drops of her martini.
The audiobook version of Cujo is read by Lorna Raver.
Cujo before he becomes rabid
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.
Despite after biting Cujo they aren't seen afterwards, this makes the bats the overarching antagonists.
In some scenes of the film, a painted Rottweiler was used since actual St. Bernard's were not capable of performing convincing simulated attacks. The Rottweiler's tail was strapped to its leg to prevent it wagging and undermining the sense of fear.
To make the dogs attack the car in the film, their favorite toys were placed in the car so they would try to get them.
There was an early draft where Cujo was also going to be possessed by a spirit of a serial killer, but it was scrapped because it was seen as too outlandish and would detract from the fear the writers wished to portray in the film.
However, it has still been theorized that Cujo was possesed by Frank Dodd, from another Stephen King book, The Dead Zone.
Despite Cujo being the main antagonist of the film, he has only 15 minutes of screen time.
