Animals are important in this book. From the beginning, we have Lennie acting like a dog and George treating him like one when he sticks his face in the green water. The imagery continues with the rabbit farm. For some reason, George and Lennie have latched on to the idea that rabbits are innocent and good and made rabbits their symbol of prosperity and the easy life.
I think that you can find many different examples of the symbolic representation of animals in the story. The animals are a way in which Steinbeck can convey aspects of both the natural world and the human one. For example, the rabbits represents the world of humans of what should be. In other words, the rabbits represent Lennie's hope for the future. One of the most constant elements in the story is how Lennie hopes for his chance to tend the rabbits. They represent his desire for something that is not of this world. They represent hope and dreams. This is why the rabbit that appears to him in the form of a hallucination at the end of the novel is a foreshadowing of the despair to come. It is at this point when Lennie realizes that he has committed a major transgression, one that will endanger his hope for the future. I think that the opposite to this would be the animals that represent the condition of the present. For example, if the rabbits that Lennie wishes to tend to represent the future, Candy's dog has to represent the present. The fact that Candy's dog is dragged out by Carlson, with no one speaking out against its murder, is a statement on how reality is. Candy's dog, a dog that has supposedly outlasted its usefulness, is taken out in the night, the "silence" as Steinbeck notes, and killed. No one, not even his owner, speaks out in defense of it before it dies. It is killed because society (the men on the ranch) want it killed. In the end, Candy's dog symbolizes the fact that the present condition in which humans live is brutal and cruel, one that represents the state of affairs in which humans live. When contrasted to the hope and zeal of the future in the rabbits, Steinbeck might be making a statement that humans have a long way to go in such a process of transformation.