The Mechanical Hound is an eight-legged robotic dog that is used to track down and kill people who may have fled after getting in trouble with the law. The Mechanical Hound tracks down people by its sense of smell and kills by projecting a sharp needle that injects a large amount of morphine or procaine
Guy Montag is threatened by the hound in the begging of book when he gets down to pet it but it growls at him and another time when he's at the fire house and slides down a pole and the hound "wakes up" and starts randomly chasing him with his needle. This foreshadows at the ending of the book when the Mechanical Hound is programmed to track down Montag.
The Mechanical Hound represents government control and how violent and cruel the government has become. In our world, dogs that serve firemen save lives by sniffing out people in need of help during fires so they can get rescued, but in this story the hound sniffs out people so they can get killed. When the Mechanical Hound is out to track down someone, the process is televised for everyone's entertainment, which shows how the people living in this dystopia are cruel and don't mind seeing someone get killed.