The Hound is a horrible robot with eight legs and a fantastic sense of smell.
The mechanical hound doesn’t sleep, and lives in a kennel in the back of the firehouse. It is, ironically, described as constantly “gently humming.” Although it is called a hound, Montag compares it to a giant bee and it has eight legs like a spider (although it does growl like a dog).
Light flickered on bits of ruby glass and on sensitive capillary hairs in the nylon-brushed nostrils of the creature that quivered gently, gently, gently, its eight legs spidered under it on rubber-padded paws. (Part I)
Of course, it is a hound because of its “olfactory system,” or its sense of smell. It is used to root out suspects. They sometimes play with it, letting it catch small animals in mere seconds. When it catches its prey, it releases a sedative. Its eyes are green-blue neon lights.
Montag tells Beatty that the Hound does not like him, and Beatty scoffs it off.
"Come off it. It doesn't like or dislike. It just `functions.' It's like a lesson in ballistics. It has a trajectory we decide for it. It follows through. It targets itself, homes itself, and cuts off. ...." (Part I)
This seems to foreshadow later in the book when Montag turns on society, favoring books to a society without them, and the Hound is set after him. Perhaps the Hound knows, before he does, that there is something different about Montag. Maybe it has been programmed to sense feelings, and sensitivity, which the other men do not have?
The Hound is a horrifying version of the bloodhound. Since it is not alive, it cannot feel. It is simply pure programming, and this makes it more heartless. It is another indication of a society that has ceased to find the joy in life. Where is the Dalmatian?
The mechanical hound is a robot shaped like a hound but has more legs. It has poison (which tries to attack Montag through this) and memorizes smells, like a real hound.
It purpose is to attack/kill those that don't follow the laws. Obviously an example of their law is reading, which is why Guy Montag got in a lot of trouble.
In Farenheit 451, the mechanical hound is a dog-shaped robot with eight legs. It is made of metal. It's purpose is to hunt down and kill fugitives, who have violated the law of the society.
After Guy Montag kills Captain Beatty, he becomes a wanted person. The hound tries to kill Montag by poison, but he manages to escape from it. "He then makes his way to Faber's house where he drinks something that changes his odor", making it impossible for the hound to find him. But the hound is somewhat programmed to kill and kills someone else thinking he is Montag.
The real purpose of the hound in this story is to show "how faceless and mechanical the system is" in our society and how free thinkers suffer in this society from bookish acts and rules.