Answer:
Mr. White begged his wife not to open the door because he was scared to see what or who is on the other side of the door. Knowing his wife had just wished for their dead son to be brought back, it is only right to assume that the dead son or his ghost had returned.
By referring to the thing outside as "it", Mr. White seems to refer to the 'non-human' nature of whatever is outside. This shows the creature may be a ghost, or a supernatural being, nothing that suggests he's normal.
Though there is no proof or anything that suggests what was outside the door, we can only assume that it was the dead son or his ghost, returning as a fulfillment of his mother's wish.
Step-by-step explanation:
The short story "The Monkey's Paw" by W.W. Jacobs revolves around the White family and their 'wishes' granted by a monkey's paw. The story deals with themes of supernatural elements, death, greed, fate, etc.
After getting hold of the monkey's paw from Sergeant-Major Morris, the white family decided to 'test' its power. They first wished for "two hundred pounds" which came true, in a way, through the death compensation from their son's employers after he died at work. The second was to bring the dead son back. It was this wish that led Mr. White to ask his wife not to open the door.
At the end of the story, though there is no proof of anything, it seemed like the son had come back, or his ghost had. So, when he asked his wife not to let "it" in, he was referring to the ghost of the son, or at whoever was knocking.
By referring to the person knocking as "it", it seems Mr. White is scared or even horrified at whatever or whoever may be at the other side of the door. He seems to be afraid of the 'being' in front of their door, knocking, fully aware that his wife had just wished for their dead son to come back to them.
Even though there is no evidence of anything or anyone outside the door, we can assume that with two wishes granted, not necessarily according to their desire, the person or being outside can be the dead son or what's left of him. With the mother's wish to get her dead son back, it seems like the dead man did indeed came back, or his ghost had. But for us to fully believe this to be true, there is nothing that suggests it actually happened. It is left to the readers to presume what happens, each to one's own conclusion.