Who is The Dog from I Am Legend and what is their importance?

The dog is a scrawny creature who apparently has escaped the scourge of the vampires by hiding. Robert befriends the dog by feeding him and gradually approaching him. Robert also has to gain trust in the dog, because it is possible that the dog is a vampire who will turn on him. Robert concludes that the dog is normal, and this discovery gives him hope that there may be human survivors. However, as Robert observes the dog's habits, he changes his mind and reflects that a dog's chances of survival may be superior to a human's because of the dog's smaller size and keener instincts.