Shadow by Aris Austin
Meet Shadow. He's a little older and slower than he used to be, but he has a heart of gold, and he's a good dog. People always tell him so. He'll never hesitate to give your face a good licking, and there's no one he loves more than his human, Brian.
Brian got a new puppy for Christmas though, leading him to abandon Shadow at the shelter he came from eleven years earlier. Shadow is devastated. Worse, the clock is winding down, even as a caring shelter worker helps him heal from the betrayal. Each day brings him closer to being euthanized in order to make more room at the shelter, and his fate rests in the hands of the very species that betrayed him—humans.