Thor

"Harrowing." Kirkus Reviews
Thor is a German shepherd belonging to a typical suburban family, wise, trustworthy, and tirelessly devoted to his human "pack." Then Uncle Ted comes to live with Thor's owners. And the terror begins. For Ted is no longer fully human, and only Thor can detect the Bad Thing lurking within Uncle Ted. Only Thor can sense the horror he is helpless to communicate. And only Thor can defeat that horror.