The mechanical hound in Bradbury's Fahrenheit 451 is also used by the firemen to intimidate and for entertainment.
The hound intimidates Montag at the station when it "wakes up" as he approaches it. It intimidates him so much that after witnessing the woman refuse to leave her burning books and be burned to death herself, he, having stolen a book and kept it, later that night, thinks the hound is outside of his home.
The firemen also use the hound for grotesque entertainment. They use the hound to destroy rodents and cats, while they watch. The hound is a nasty "beast."
The purpose of the mechanical hound is to chase down and drug people who have been identified as dangerous to the government in this book.
The way it works is that the hound is programmed with the person's scent and such (that's why Montag drinks the stuff that will change his scent). It has sensors that will then home in on that person and that person only. It has a dart that it shoots out, drugging the person.
We are told that the hound has eight legs and that it uses morphine or procaine on its victims.