What are some of the animals in the book Of Mice and Men?

In John Steinbeck's Of Mice and Men, animals provide rich symbolism throughout the novel. While the setting of the novel, rural California, provides an appropriate landscape for many animals, the most significant include mice, rabbits, and dogs. Steinbeck repeats these images throughout the novel, allowing the reader to examine the hierarchy of strength among both animal and man, and the idealized American dream. 

Early in the character's development, Steinbeck introduces the relationship Lennie has with animals, particularly small, soft, innocent animals. George catches Lennie with a dead mouse in his pocket and throws it into the brush. 

Lennie explains that his Aunt Clara used to give him mice to play with, but that she isn't around anymore. After the death of Aunt Clara, George promised to take care of Lennie, which likens Lennie's relationship to George to that of an animal. Although he is large in size and possesses physical strength, Lennie does not have the skills necessary to survive on his own and completely depends on George for his well-being. At the end of the novel, George must do what he feels is best for Lennie and shoots him. Steinbeck creates a parallel here between George's actions and that of an individual putting an animal down if that animal would not have a good quality of life. (Lennie is more than a pet, though; he offers George purpose and is an important part of the future they dream of.)

While George acknowledges that it would be easier for him to live his life alone, working on farms and surviving by himself, Lennie provides companionship for George. Without Lennie, George would not have as meaningful a purpose for working towards their dream of owning their own farm. In this capacity, Lennie cares for George in a meaningful and important way as well.

Furthermore, regarding the hierarchy of strength, Steinbeck creates a parallel between Candy and his elderly dog. Once useful and strong, Candy's dog is now old, lame, and physically unappealing. While Candy's dog no longer proves to be helpful on the ranch, he provides Candy with companionship. Despite this relationship, Candy allows Carlson to put his dog down with a Luger pistol and regrets that he did not carry out the deed himself. Candy is no longer strong or as able as he once was, and therefore not as useful on the ranch. He fears that as a weaker individual, he will no longer be welcome on the ranch and offers his life savings to George and Lennie to accompany them on their farm. 

George and Lennie's dream of owning their very own farmhouse on a few acres of land is closely tied to rabbit imagery throughout the novel. George dreams of a life in which he and Lennie can sustain themselves and live a simple, yet safe life; Lennie dreams of being able to care for the rabbits that he loves. In the wild, rabbits are very difficult to catch; they are fast and agile. Throughout the novel, George and Lennie desperately try to get their hands on their interpretation of the American dream, but always come up just a little short. Ultimately, right before his death, Lennie hallucinates a giant talking rabbit. This final image speaks to the overwhelming difficulty, perhaps impossibility, of actually attaining the American dream after all.