In Chapter 10, Tim Johnson is the liver-colored bird dog belonging to Mr. Harry Johnson who drives a bus; Tim Johnson roams the streets while his owner is gone. One day Jem watches this dog who acts abnormally; concerned, he asks Calpurnia to take a look at him. When she espies him, the dog walks erratically; she grabs the children and hurries them home where she phones Atticus, telling him there is a mad dog on their street.
When Sheriff Tate arrives with Atticus, Heck Tate insists that Atticus use the rifle. Shocked by his expertise, the children see their father shoot the dog in the head, killing him instantly. Unbeknowst to the children, Atticus has been known as "Ol' One Shot."
Later, when Tom Robinson is shot as he tries to escape from the Maycomb jail, the memory of Tom Robinson is recalled. For, Tom is shot dead, and his fears mean nothing, just as those of the dog mean nothing. Thus, Tim Johnson becomes symbolic of the damaged innocent who is sacrificed for the society.
Tim Johnson is a neighborhood dog. At first the reader may find it strange that a dog has a name and last name, but this seems to be a common practice in Maycomb, as the dog that belongs to Judge Taylor also has a name and last name. However, the author makes an interesting play on words with the dog's name, Tim Johnson, and the name of the victim at the epicenter of the conflict in Maycomb, Tom Robinson.
At least in Scout's memories, the two names became correlated and intertwined, after all both Tim and Tom suffered similar fates.
In Tim the dog's case, his wondering around caused him to get rabies. This is why suddenly the dog begins to act strangely, to the point that Calpurnia recognizes the signs and takes the kids to a safe and isolated location.
"She [Calpurnia] followed us beyond the Radley Place and looked where Jem pointed. Tim Johnson was not much more than a speck in the distance, but he was closer to us. He walked erratically, as if his right legs were shorter than his left legs. He reminded me of a car stuck in a sandbed."
When Atticus learns about it, he takes his rifle and goes to shoot the dog at close range to put him out of its misery quicker. This was a moment that shattered most of Scouts's innocent view of life. When Tom Robinson tries to escape the lynch mob and is shot, just like the dog, he becomes a casualty of a battle between worlds that will never find a true ending. He is the innocent victim that is mercifully put down, just like the dog, before others get to him and do something even more vile and torturous.