The Mechanical Hound is the government's technological enforcement. Like a real dog, the hound can sniff out things; in this case it is used to sniff out books. Using a sense of smell rather than visual monitoring, the hound is still a means of surveillance, giving new meaning to the term "watch dog." The hound also represents the oppressive use of technology. In this way, the hound functions like the parlour shows. The shows are a technological advancement that have become used to create a passive thoughtless public. The hound is used to hunt and kill those who attempt to challenge their passive, oppressed lives. Both enforce the government's agenda.
In trying to create a public that does not think critically, a public that goes through the motions of their lives mechanically, the government in Montag's world is essentially trying to make the public into a thoughtless machine. This is what the Mechanical Hound is. Although it is described as living and not living, it is a machine. As Beatty says:
"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. It's only copper wire, storage batteries, and electricity."
The government uses the hound to enforce its policies. The government wishes to command the public as it commands the hound. The government uses the hound to enforce the "trajectory" they have for the entire society. This shows an interesting intersection of using technology to control life. With such stringent control, the effect is that society becomes like a machine, like the hound: thoughtless and less sentient.