In Fahrenheit 451, how does the hound react to Montag?

The mechanical hound targets Montag, as if it were suspicious of him.

Montag does not like the mechanical hound.  He finds it positively creepy.  The mechanical hound is a robot that seems neither living nor dead, an animal paradox.  It is used to track and target law-breakers and book-hiders.  Montag has secrets, and he fears that hound knows it.

It was like a great bee come home from some field where the honey is full...

The mechanical hound targets Montag, as if it were suspicious of him.

Montag does not like the mechanical hound.  He finds it positively creepy.  The mechanical hound is a robot that seems neither living nor dead, an animal paradox.  It is used to track and target law-breakers and book-hiders.  Montag has secrets, and he fears that hound knows it.

It was like a great bee come home from some field where the honey is full of poison wildness, of insanity and nightmare, its body crammed with that over-rich nectar and now it was sleeping the evil out of itself. (Part I)

The firemen like to play with the hound, setting it on rats and betting on it.  Montag doesn't participate in these games because his wife Mildred forbids it.  He still does not like the hound, especially when he has a guilty conscience from stealing the book.

The other men laugh at Montag for fearing the beast, which is not really a beast.  They tell him it has programming, not instincts.  Montag thinks the hound does not like him.  It seems it might be able to smell fear.  The mechanical hound attempts to attack Montag when he touches its muzzle.

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. (Part I)

The hound possesses a stinger, which contains a sedative of cocaine or morphine.  The hound seems like it is trying to sting Montag.  Despite the protestations that the hound is not a danger to him, Montag is suspicious of it for seeming to be suspicious of him.

The hound’s interactions with Montag, and Montag’s fear and unease around it, foreshadow his later period as an outlaw.  He goes from being a fireman for whom the hound works to a target of the hound’s aggression when he kills Beatty and goes on the run.