Animal imagery is used in at least two ways in this book. In both cases we can interpret the imagery as having symbolic value or representational quality.
The most meaningful animal images are those which are repeated. Lennie is connected to mice and rabbits, innocent yet furtive creatures, soft and defenseless but wily and invasive as well.
Throughout the novel, Steinbeck associates Lennie with animal movements, and Lennie’s love of dogs and rabbits brings about the tragedy of the novel.
Lennie's inability to become truly socialized can be seen in the repeated connection between his character and animals. Like the rabbits he dreams of and the mice he keeps to pet, Lennie is wild at heart, unable to follow social rules.
Candy is also connected to an animal, his old, smelly dog. Very much like his dog, Candy has nothing to offer anyone but a small amount of comfort. This is true of Candy's suggestion that he join Lennie and George in buying a piece of land. In this case Candy is merely padding the dream; making it seem more realistic.
Candy's dog was his only companion, but the dog was not a real companion in the human sense. The dog needed to be cared for and tended, like George and Lennie's dream, any comfort it offered was intangible and ultimately misleading.
Animals also figure into the setting of the book, appearing as part of the opening section. Here, animal imagery is used to convey an idyllic landscape: quiet; pastoral; timeless; natural. Living a "natural" life is not possible for George and Lennie. They are part of a commercial, capitalist system that makes the rules for their lives.
The animals represent a natural order which Lennie craves and, ultimately, belongs to but which is denied to all the characters in the book.