Candy's dog symbolizes what happens when something gets too old to serve the purpose that they used to. Candy's dog was once a very good sheepdog, but now it is weak due to age. Because the dog no longer serves a purpose, it is killed. Candy fears that this will be his own fate. When he becomes too old and no longer useful, he fears that he will be kicked off the ranch and have no where to go.

Candy's Old Dog