In John Steinbeck's Of Mice and Men, what happens with Candy and his old dog foreshadows what happens between George and Lennie at the end of the novel. Candy's dog is at the point in its life when it is worn out with old age and various ailments. Thus, Carlson urges Candy to put the dog out of its misery. Candy cannot bring himself to do it, though, because he has had the dog since it was a puppy. Given Candy's attachment to the dog, Carlon shoots the dog in Candy's stead.
Like Candy's old dog, Geoge and Lennie have been together for a long time. At the end of the novel, Lennie is facing imminent death at the hands of the lynch mob. Whereas Carlson puts Candy's dog out of his misery, Steinbeck has George himself kill Lennie before the lynch mob can kill Lennie. George even uses the same weapon as Carlson:
He reached in his side pocket and brought out Carlson’s Luger; he snapped off the safety, and the hand and gun lay on the ground behind Lennie’s back. He looked at the back of Lennie’s head, at the place where the spine and skull were joined.
Some similarities between Candy and his dog are that they are both old and will probably be "kicked out" of the ranch as soon as they are not useful. Some similarities between George and Lennie are that they are male and human. There is not much similar between them: George is small and smart, while Lennie is huge and strong but has mental problems.