InFahrenheit 451the Mechanical Hound is trained to sniff out people who read books and who think for themselves as well as seek out the location of hidden books. It has the ability to track people by their scent and can kill them because of their crimes. When Montag runs after killing Beatty, he drinks a mixture that prevents the Hound from finding him.
The animals were turned loose. Three seconds later the game was done, the rat, cat, or chicken caught half across the area way, gripped by gentling paws while a four-inch hollow steel needle plunged down from the proboscis of the Hound to inject massive jolts of morphine or procaine.
The Mechanical Hound is significant in that it shows the destructive nature that technology has evolved into. The hound is described as evil. Unlike a normal dog, he is said to have eight legs and shoots morphine and procaine into the body of those who he catches.
The Hound also shows how brutal the government has become. The laws and protections we are accustomed to are long gone. There is no court system or legal proceedings, there is just the Hound who has been programed by the government to find, hunt, and kill citizens who break the law.