Fahrenheit 451 is a 1953 dystopian novel by Ray Bradbury, about a future world where television is the primary form of entertainment and books are banned as subversive.
Montag is a fireman -- in this world, a fireman is someone who burns books, not someone who fights fires. At the fire station, there is a Mechanical Hound, which is used to root out fugitives and book collections; the Hound is also used for entertainment by the other firemen, who bet on its prowess in killing animals. The Hound has a poison needle in its nose. Near the beginning of the book, the Hound seems suspicious of Montag, and threatens him; later, when Montag flees, another Hound pursues him and the spectacle is broadcast on live TV for citizens to enjoy.
During Montag's crisis of faith in the system, he returns to the firehouse in the middle of the night. When he arrives, he finds that:
The Mechanical Hound was gone. Its kennel was empty and the firehouse stood all about in plaster silence ... and Montag came in through the silence and touched the brass pole and slid up in the dark air, looking back at the deserted kennel, his heart beating, pausing, beating.
(Bradbury, Fahrenheit 451, hrsbstaff.ednet.ns.ca)
The Hound is gone because Chief Beatty has sent it to Montag's house, to wait for them. Chief Beatty knows that Montag has been collecting books, and he also knows that Montag is scared of the Hound. He wanted to use the Hound to make Montag scared, to make him renounce his curiosity; in the end, Montag destroys the Hound and flees.