The Mechanical Hound is an eight-legged robotic animal used to hunt down intellectuals and political dissidents in Bradbury's dystopia.
The Mechanical Hound is capable of remembering ten thousand odor indexes and is programmed by Captain Beatty to track down its prey like a bloodhound. The Hound moves quickly, never misses a target, and immobilizes its prey by injecting them with a four-inch needle of procaine or morphine, which protrudes from its proboscis. It is a sleek, intimidating piece of advanced technology that hums with electricity and has "green-blue neon" eyes.
The Mechanical Hound functions on command and begins to worry Montag when it growls at him in the station. Captain Beatty suspects that Montag is interested in literature and programs the Hound to sniff outside of his home as a warning.
Later in the story, Mildred calls in an alarm on Montag and he is forced to destroy his home using a flamethrower. Montag then kills Captain Beatty and narrowly escapes the Mechanical Hound at the last moment. However, the Hound manages to inject Montag with procaine, which completely numbs and paralyzes his leg. While Montag is on the run, the authorities fly in another Mechanical Hound to track him down. After Montag evades the police and reaches the river, the traveling intellectuals watch from a portable television as the second Mechanical Hound seizes another unsuspecting victim.