Throughout the novel "Of Mice and Men" we come across certain parts in the book that gives us, the reader an insight into the ending of the story. We come across the death of Candy's dog that marks a major omen in the story. There are also some quotes that are like parallels to the end of the story, although the reader can interpret them as bad omens, the characters have no idea.
Therefore, I think that the dog is a symbol of the treatment meted out to the old or those considered inconvenient or no longer of any use. I believe, this is an obvious metaphor for what George must do to Lennie, who proves to be no good to George and no good to himself. Steinbeck re-emphasizes the significance of Candy's dog when Candy says to George that he wishes someone would shoot him when he's no longer any good.
And when Carlson ends up shooting the dog, Lennie is the only man not inside the bunk house, I think Steinbeck placed him outside with the dog, away from the other men, to somehow put him in or show the position he will be in at the end of the story. He also makes use of the same gun at Lennie's end; the only difference is that George is the one holding the gun.
I also think the implication of the omens in of mice and men are that the death of Candy's dog is a direct parallel, by this I mean that it almost describes in detail what happens to Lennie at the end. For example, the dog is shot in the back of the head; Lennie too is shot in the back of the head. I found out from reading "of mice and...