Opening and Last Scene;
At the start the Sun is shining, because it is the start of something new. At the end the Sun is dull and faded, as everything is coming to an end.
Rabbits are present in the first chapter because Lennie is there and he's happy. The rabbits are not there in the last chapter because Lennie is about to be killed.
In chapter 1 the Heron had life and power, it showed dominance. Which contrasts with Lennies physical strength. In chapter 6 the Heron sat there waiting motionless which builds tension. This is because Lennie is at the bottom of his strength, he is now weak.
Lennie's puppy represents the victory of the strong over the weak. Lennie kills the pup accidently, by virtue of failure to recognise his own strength. The huge Lennie was soon to meet fate similar to that of his small puppy. Like an innocent animal, Lennie is unaware of vicious, predatory powers that surround him.
Candy's dog represents the fate awaiting someone who has outlived their purpose. His dog was a fine, useful dog on the ranch and debilitated by age. This could foreshadow the life of Candy, when he becomes to weak to work and is no longer welcome. Candy raised the dog from a pup, but Carlson insisted to kill him painlessly. Candy feels guilty that he did not shoot the dog himself. This teaches George a lesson. The event foreshadow's George having to shoot Lennie. It seemed right that George knew how to handle his death and he felt more comfortable with it.