Which Stephen King novel features a killer St. Bernard dog? The answer is Cujo.
Published in 1981, Cujo follows a normally friendly St. Bernard who contracts rabies after being bitten by a rabid bat. The infection transforms the dog into a deadly threat for the Trenton family, particularly Donna and her young son Tad, who become trapped in a car outside a rural auto-repair shop.
The story is set in Castle Rock, Maine, one of King's recurring fictional communities. King said the premise was partly inspired by an unsettling encounter with a large Saint Bernard at a Maine mechanic's shop, along with a newspaper report about a child killed by a Saint Bernard. The novel's terror comes from disease, heat, isolation, and mechanical failure rather than a supernatural monster.
Readers sometimes remember the 1983 film more clearly than the novel. The movie changes important plot details, including the ending, so its version should not be treated as a complete substitute for King's book. Cujo won the British Fantasy Award for Best Novel in 1982.