Cujo

Once upon a time, not so long ago, a monster came to the small town of Castle Rock, Maine.... He was not a werewolf, a vampire, a ghoul, or an unnameable creature from the enchanted forest or snow wastes; he was only a cop....
Cujo is a huge Saint Bernard dog, the best friend Brett Camber has ever had. Then one day Cujo chases a rabbit into a bolt-hole. Except it isn't a rabbit warren anymore. It is a cave inhabited by rabid bats. And Cujo falls sick. Very sick. And the gentle giant who once protected the family becomes a vortex of horror inexorably drawing in all the people around him....