He fears the mechanical hound. When he goes to the firehouse early in the story, Montag touches it muzzle and it growls at him and extends its needle an inch and then pulls back. Montag tells Captain Beatty "It doesn't like me." (pg 26) Captain Beatty thinks he is being silly but Montag feels that
"It would be easy for smeone to set up a partial cmbination on the Hound's memory, a touch of amino acids, perhaps. That would acocunt for what the animal did just now. Reacted toward me." (pg 26)
That was the third time it had threatened Montag.
He wonders what the Hound thinks about at night. Beatty tells him it doesn't think about anything because they don't want it to think and they program it. Montag says that is sad because all they program into it is "hunting and finding and killing. What a shame that is all it will ever know." ( pg 27)
Later in the novel, when Montag shows Mildred his books, Montag and Mildred hear a faint scratching at the font door. Then a "slow, probing sniff, an exhalation of electric steam" (pg 72) When Mildred identifies it as a dog, she asks if Montag wants her to shoo it away. Montag says "Stay where you are!" (pg 72) He won't let her open the door.
When Montag is taken to his own home to burn it, Faber asks him "Can't you run, get away?" Montag replies, "No, The Hound! Because of the Hound! (pg 116) His fear is very real.
After Montag kills Beatty, the Hound attacks him. It leaps at him and he blasts it with the flame gun. It stabs him in the leg before the flames take over and it blows up. (pg 120)