The Mechanical Hound is a tool used by the firemen to hunt and kill dissidents. With this tool, people can be tracked by their biochemistry and so the firehouse does not need to expend manpower. The Hound represents one way technology is adapted to become harmful instead of beneficial; the Hound is a marvel of engineering but is used only to kill. Montag is inherently wary of the Hound, because he knows how easily it can kill:
"No, no, boy," said Montag, his heart pounding.
He saw the silver needle extended upon the air an inch, pull back, extend, pull back. The growl simmered in the beast and it looked at him.
(Bradbury, Fahrenheit 451, Google Books)
The Hound represents everything brutal about the government of the future. Instead of due process, dissidents are hunted and killed for the entertainment of the masses. The Hound has no conscious mind, just programming, just like many of the citizens. They have been programmed by television; the Hound is programmed by the government, and when the government feels that a citizen has altered or diverted from his programming, the Hound comes out to kill.