In the book there is a lot of foreshadowing going on. For example, Lennie's kills get more important as the book goes on, from a mouse to a pup to Curley's wife, showing a clear pattern. Another is when Candy's dog is shot, which foreshadows the ending where George shoots Lennie, but one note must be that Candy says he should have shot the dog himself, so this applies to George in the end, as he copies the action onto Lennie. Another example of foreshadowing is George tells Lennie that if he gets into trouble to come back and hide in the bush, this tells us that most definitely Lennie will get into trouble. Also, George warns Lennie of Curley's wife and tells Lennie that she will cause trouble, this leads to Curley's wife causing trouble for Lennie and all the men. The men refer to Curley's wife as a rat trap and Lennie's love for mice leads to the accidental killing of Curley's wife for Lennie kills all the mice he's ever owned.
An interesting example is Candy's dog, suggestive of the senescent American dream that wants rejuvenation (symbolized by one of Slim's pups). Candy's dog also is symbolic of his love and friendship towards another animal. In a way, the weak old dog is a reflection of Candy and his old, handicapped state. The death of Lennie and Candy's dog seem similar. Both seem to have a test to spare the one's they love. Candy must spare his dog from the pain of staying alive. George must spare Lennie from the wrath of Curley.