The rabbits in Of Mice and Men are very important to Lennie. He is a gentle-hearted creature that longs for soft cuddly animals the same way a small child loves a favorite stuffed animal. Unfortunately, Lennie is unable to control his considerable strength or to show restraint. The small animals he pets (the mice and the puppy) end up dead. The rabbits in the story are a metaphor for Lennie's dream for the future.
The rabbits in Lennie's mind represent a paradise. Like children wanting to hear about Christmas, Lennie listens to George talk about the farm as a place where they both can be happy. If he is good, he will live on the farm with George and have rabbits as friends. However, if he does "a bad thing" he worries that this dream will not come true and George will not let him take care of the rabbits. Such is Lennie's simplistic view of the world.
At the end of the story, when Lennie kills Curley's wife, paradise has been lost. The giant rabbit appears to Lennie, at the pool, and chastises him for being bad. This is a nightmarish reversal of Lennie's dream. The thing that has given him comfort in the past is now telling him all of his dreams have ended. Though Lennie does not comprehend that he might be lynched, the loss of his paradise is devastating.