What makes the Mechanical Hound even more frightening, is the fact that he searches for his prey by scent, but eventually can be fooled into killing another man, not Montag. Montag watches on TV a man on the street identified as himself and killed by the Mechanical Hound. Montag has given up his identity to stay alive.
When Montag is trying to run away from the second Mechanical Hound, he changes his clothes, he puts on Faber's clothes. But the Hound still hunts him. Montag meets the homeless men and Granger gives him a liquid to alter his scent, technically he is altering his chemistry, his biology to prevent the Hound from finding him. The scary thing is that the Hound murders someone else so that the police won't have to admit that they lost Montag's trail.
The sweet firehouse dog has been changed into a killing machine.
In Montag's world, a Mechanical Hound made of brass, copper, and steel has replaced the usually gentle and friendly firehouse dog of our society. The menacing creature has "a four-inch hollow steel needle" in its mouth which it uses to inject lethal doses of morphine or procaine into its victims. The hound's purpose is to track down people who are harboring illegal books. It is also sometimes used by the firemen for illicit, sadistic gambling games in which chickens, cats, or rats are released in its presence and bets are taken on which will be the Hound's first victim.
The Hound usually lies dormant in its kennel, and comes to life with a "green-blue light flickering in its...eye bulbs" and a growl that is "a strange rasping combination of electical sizzle, a frying sound, a scraping of metal, a turning of cogs that seem(s) rusty and ancient with suspicion". It is a terrible machine, symbolic of a society where mechanization is more powerful and important than humanity.