Cujo is the titular antagonist in the novel and movie of the same name Cujo.
Cujo was once the beloved St. Bernard of the Camber family, but was bitten by a rabid bat in the nose when his head got stuck in a bat cave while chasing a rabbit (which was shown at the beginning of the movie).
Cujo then went mad with rabies and killed any person he came across including one of his own owners.
In the end of both the book and movie, Cujo was seemingly killed by Tad's mother, Donna (who also tried to save her son from dying of heatstroke), by being shot by the sheriff's gun. In the novel he is killed after being hit by a baseball bat.
Cujo appears to have survived the gun shot and makes a cameo appearance in the film Cat's Eye, where he is shown chasing the film's titular cat. (Or it could be another dog of the same breed.)