Personification attributes human characteristics to non-human objects or animals.
Throughout the novel, Montag associates malevolent human characteristics with the Mechanical Hound. The Hound, as the word "mechanical" implies, is a machine shaped to look like a hound. It aids the firemen with their book burning. As a mechanical object, it can have no real emotions and is neither good nor evil. However, from the start of the novel, Montag relates to it as if it is alive. For example, he watches it "sleeping the evil out of itself."
It's not difficult to see how Montag could connect the Hound with human evil. The Hound's job is to pounce on enemies, rodent, human, and otherwise. It injects them with a hypodermic needle full of morphine or procaine. We associate the act of giving injections with human beings.
Montag states that the Hound "doesn't like [him]," as if it had human feelings.
Later on, after Montag shoots the attacking Hound, he continues to personify it as an entity that hates him on a personal level:
Montag lay watching the dead-alive thing fiddle the air and die. Even now it seemed to want to get back at him and finish the injection which was now working through the flesh of his leg.