The mechanical hound is clearly described as a deadly predator designed specifically to hunt down and immobilise prey. When we are first introduced to the mechanical hound that is kept in the fire station, we are told how to amuse themselves the firemen would let loose rats or other animals in the firestation compound and watch the ensuing carnage:
The animals were turned loose. Three seconds later the game was done, the rat, cat, or chicken caught half across the areaway, gripped by gentling paws while a four-inch hollow steel neeedle plunged down from the proboscis of the Hound to inject massive jolts of morphine or procaine.
Of course, we see how the hound operates with a human at the end of the story, when the hound catches a man who is depicted to be Montag and leaps up into the air, before seizing this poor figure and injecting him in the same way that the animals are injected and killed.
The mechanical hound can inject lethal jolts of morphine or procaine from a four-inch hollow steel needle to kill a human. *
*for further reference check out pg. 25 of Fahrenheit 451
The hound can track down humans by following their sceent trail. When the hound tries to find Montag, the dog cannot becuase the people at the train tracks gave him somethign that would chage his scent. This would make it so that the hound could never find him. Hope this helps :]