Well, consider each animal. There's a water snake in the first chapter. It disappears. In the last chapter, we see another, and it gets gobbled up by a bird.
Lennie has a mouse in his pocket. He has accidentally killed it.
Lulu had pups, but Slim drowned the smallest.
Candy's old dog is shot dead.
Lennie is very upset by the notion that some cat might get his rabbits.
Lennie sees an imaginary rabbit when he's hiding in the brush waiting for George.
The horses and mules rattle their chains every time someone enters or exits the barn.
Lennie kills the pup that Slim gives him.
Use these to develop a thesis statement about the treatement of animals. Then relate that to the treatment of people in the novel.