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 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.

In Fahrenheit 451, how does the hound's hatred for Montag become evident at the end?

At the beginning of "Fahrenheit 451," the first time that we are introduced to the hound, it expresses a rather disturbing and illogical hostility towards Montag.  It is just a machine, one who has to be programmed with DNA or some form of scent or fluid of a person before it learns to "hate" or hunt or stalk or destroy anyone.  And yet, as Montag comes into the fire station at the beginning of the book, "the hound growled" and Montag

"saw the silver needle extend upon the air an inch...the growl simmered in the beast and it looked at him."

The hound, a machine, seems to instinctively consider Montag to be an enemy, and is even stepping forward menacingly as Montag slips upstairs.

This event actually foreshadows the hound's chase of Montag at the end of the book.  Indeed, Montag does turn out to be the enemy, and it chases Montag through the city streets, intent on the kill.  Perhaps it did sense Montag's defection even before he himself realized it, who knows.  Either way, Bradbury uses the hound's reaction at the beginning as a form of foreshadowing for the ending of the book.  I hope that those thoughts helped; good luck!

In Fahrenheit 451, what could be another reason for the hound's reaction to Montag?

This could be Bradbury's way of foreshadowing Montag's interest in books and his opposition to the firemen and the Mechanical Hound. But we can speculate that the Hound was reacting to how it was originally programmed. In other words, the Hound was reacting to the scent or sense of the chemicals which make up the physical material of a book, or of some other suspicious object or person. 

Maybe the Hound is so advanced that it can sense pheromones or Montag's anxiety and the Hound is able to calculate that this anxiety stems from a conscious or subconscious interest in books and freedom of thinking. In other words, perhaps Montag's brief encounter with Clarisse had already begun to change the chemical reactions in Montag's mind and the Hound is even sensitive to this.

The most plausible explanation probably does come from his encounter with Clarisse. She touches him with the dandelion. There are three possibilities. The Hound is picking up on Clarisse's DNA which had rubbed off on Montag. She is already regarded by the authorities as a suspicious free thinker. O, the Hound is picking up on some scent of a book which was passed from Clarisse's hand to Montag's chin. Or, the Hound is picking up on the scent of the dandelion itself. Since most people spend their time glued to the parlour shows and ear implants, maybe no one experiences or even touches nature. The Hound might be set to be suspicious of anyone with traces of nature.