The mechanical hound is a super-fancy robot machine that injects a poison into anyone how is considered "dangerous" to Montag's society. It is highly advanced; its sense of smell is much better than the best dog's sense of smell, so it is excellent at tracking people down. The novel states that on long nights at the firehouse, they would "let loose rats in the firehouse...sometimes chickens, and sometimes cats...and there would be betting to see which...the Hound would seize first." So, they played betting games on which animal the hound would kill first.
As Montag reaches for the hound in the beginning of the novel, it "growled...[and] half rose in its kennel and looked at him...[and] took a step from its kennel" with its needle raised. This scares Montag (as it should) and he questions Beatty as to the possiblity of his own DNA codes being programmed into the hound, or if the hound could possibly be remembering Montag from previous burning excursions. Beatty blows it off saying, "It doesn't like or dislike. It just functions." The hound reacting aggressively becomes more significant later as it is dispatched to Montag's house to "scare" or "check up" on him, and then later as Montag himself is hunted by it. So, Montag's fears and intuitions were right on; it didn't "like" him, it was being sent after him, and he has every reason to fear it.