The Mechanical Hound is an impressive piece of technology used by the fireman institution to track down political dissidents and incapacitate them. In Part One, Montag arrives at work and walks past the menacing Hound, which is silently resting. Bradbury describes the Mechanical Hound by writing,

The Mechanical Hound slept but did not sleep, lived but did not live in its gently humming, gently vibrating, softly illuminated kennel back in a dark corner of the firehouse ... 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. (22)

Montag is fascinated by the creature but is overcome with an unsettling feeling when he looks at the Hound in the corner of the firehouse. Bradbury then elaborates on how the firemen pass their time on boring nights in the firehouse by writing,

Nights when things got dull, which was every night, the men slid down the brass poles, and set the ticking combinations of the olfactory system of the Hound and let loose rats in the firehouse area-way, and sometimes chickens, and sometimes cats that would have to be drowned anyway, and there would be betting to see which the Hound would seize first. (22)

Shortly after the firemen let animals loose, the Hound quickly pounces on the nearest prey and injects the defenseless animal with morphine or procaine from the large, sharp needle protruding from its nose. Lately, Montag has not been participating in the games with the other firemen. After staring at the resting Hound, Montag climbs to the upper level of the firehouse and tells Captain Beatty that the Mechanical Hound does not like him. Beatty responds by telling Montag,

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. (23)

As the novel progresses, Montag witnesses a woman commit suicide during a routine call and begins to wonder what is inside books that is so powerful. Montag thinks about the stolen novels he has hidden inside his ventilator and acknowledges that he is trapped in an unhappy marriage. After Mildred informs him that Clarisse is dead, Montag looks out the window and can sense the presence of the Mechanical Hound. Bradbury writes,

Outside the house, a shadow moved, an autumn wind rose up and faded away but there was something else in the silence that he heard. It was like a breath exhaled upon the window. It was like a faint drift of greenish luminescent smoke, the motion of a single huge October leaf blowing across the lawn and away. The Hound, he thought. It's out there tonight. (45)

Eventually, Montag is accused of being an intellectual, and the Mechanical Hound is used to track him down. Montag manages to kill the Mechanical Hound and flees the dystopian city.

The Mechanical Hound symbolizes all that is wrong with the highly technological society in which Montag lives: the technology that is supposed to serve works against humans and other living creatures, not for them. The Mechanical Hound's purpose is to attack the people the state has decided are its enemies.

The Mechanical Hound is very creepy, much like a figure from a horror story. It is always associated with cold-blooded death. We learn that the firemen will let loose animals like rats, cat, and chickens in the firehouse and bet on which one the Hound will kill first. In this mechanized world, natural creatures don't stand much of a chance. They are reduced to becoming the pawns of technology:

The animals were turned loose. Three seconds later the game was done, the rat, cat, or chicken caught half across the areaway, gripped in gentling paws while a four-inch hollow steel needle plunged down from the proboscis of the Hound to inject massive jolts of morphine or procaine. The pawn was then tossed in the incinerator.

Although he understands it has no consciousness, Montag tends to personify the Hound, perceiving it as actively malevolent. Early in the novel, for example, it seems to be trying to attack him:

The Hound half rose in its kennel and looked at him with green-blue neon light flickering in its suddenly activated eyebulbs. It growled again, a strange rasping combination of electrical sizzle, a frying sound, a scraping of metal, a turning of cogs that seemed rusty and ancient with suspicion. "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.

Montag says to Beatty:

"what does the Hound think about down there nights? Is it coming alive on us, really? It makes me cold."

Later in the book, the Hound again seems alive to Montag and appears to want to harm him:

He turned and the Mechanical Hound was there. ... It made a single last leap into the air, coming down at Montag from a good three feet over his head, its spidered legs reaching, the procaine needle snapping out its single angry tooth. Montag caught it with a bloom of fire .... Montag lay watching the dead-alive thing fiddle the air and die. Even now it seemed to want to get back at him and finish the injection which was now working through the flesh of his leg.

The Mechanical Hound is an important part of the story, and therefore, we can find lots of quotations about it in the book.

Firstly, we can get an idea of its physical characteristics and appearance from its introduction to the reader when Montag is at the firehouse:

The dim light of one in the morning, the moonlight from the open sky framed through the great window, touched here and there on the brass and the copper and the steel of the faintly trembling beast. 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. 

From this same part of the text, the reader also gets a glimpse of how the Mechanical Hound attacks its prey. This quotation comes from Montag's description of a game that the fireman play in which they set the hound on a group of animals, usually chickens or cats, and watch the ensuing chaos:

Three seconds later the game was done, the rat, cat, or chicken caught half across the areaway, gripped in gentling paws while a four-inch hollow steel needle plunged down from the proboscis of the Hound to inject massive jolts of morphine or procaine.

In addition, in the following quotation, we see how the hound reacts to Montag. For Montag, this is a terrifying experience that makes him believe that the hound doesn't like him:

The Hound half rose in its kennel and looked at him with green-blue neon light flickering in its suddenly activated eyebulbs. It growled again.

Finally, in this quotation, Montag describes what it feels like to be pursued by the hound, as he experiences in the final part of the book:

He could feel the Hound, like autumn, come cold and dry and swift, like a wind that didn't stir grass, that didn't jar windows or disturb leaf-shadows on the white sidewalks as it passed. The Hound did not touch the world. It carried its silence with it, so you could feel the silence building up a pressure behind you all across town.