In Edgar Allan Poe's "The Black Cat," what does the second cat (with the patch on his neck) remind the narrator of?

The
narrator of 's "" explicitly states that the second cat reminds him of Pluto, the
first cat that he mistreated and killed. However, as the narrator spends more time with the
second cat, it also begins to represent certain incorporeal aspects of the narrator's own
character.

Firstly, the two cats share a number of similarities.  Like Pluto,
the second cat shows a fondness for the narrator.  To his surprise, the narrator soon finds
"a dislike to it arising within [him]" due to this.  He continues,

By slow degrees, these feelings of disgust and annoyance rose into
the bitterness of hatred. I avoided the creature; a certain sense of shame, and the remembrance
of my former deed of cruelty, preventing me from physically abusing it. I did not, for some
weeks, strike, or otherwise violently ill use it; but gradually -- very gradually -- I came to
look upon it with unutterable loathing, and to flee silently from its odious presence, as from
the breath of a pestilence.

Because the cat reminds the
narrator of Pluto, it reminds him of what he did to the first cat as well.  This connection is
furthered by how nearly identical the new cat is to Pluto, as the very next day after the
narrator brings the cat home he discovered "that, like Pluto, it also
had been deprived of one of its eyes."  With the exception of a patch of white on its
chest, it is nearly identical to Pluto.  The wound elicits sympathy in the narrator's wife,
which eventually contributes to the wife's murder later in the story.  The cat continues to show
strong affection toward the narrator, and he grows more and more angry at this, calling its
caresses "loathsome."  He feels the desire to kill it, just as he killed Pluto, but is
stopped by his "absolute dread of the beast."  Unlike with Pluto, and perhaps even
because of what he did to Pluto, the narrator feels afraid of the creature.

There are a number of reasons why the narrator is reminded of Pluto.  However, as the
story progresses, the narrator begins to focus in on the one discernible difference between the
cats, stating:

My wife had called my attention, more than
once, to the character of the mark of white hair, of which I have spoken, and which constituted
the sole visible difference between the strange beast and the one I had destroyed. 

Over time, this patch begins to take on a more specific form in the
narrator's mind:

The reader will remember that this mark,
although large, had been originally very indefinite; but, by slow degrees -- degrees nearly
imperceptible, and which for a long time my Reason struggled to reject as fanciful -- it had, at
length, assumed a rigorous distinctness of outline. It was now the representation of an object
that I shudder to name -- and for this, above all, I loathed, and dreaded, and would have rid
myself of the monster had I dared -- it was now, I say, the image of a hideous -- of a ghastly
thing -- of the GALLOWS ! -- oh, mournful and terrible engine of Horror and of Crime -- of Agony
and of Death!

Through the narrator's meticulous focus on
the mark, it eventually takes on the shape of the gallows in his mind.  Like many of Poe's
narrators, the man is most likely projecting.  In this case, the narrator is projecting his
feelings of guilt for his previous crime, and perhaps even for the crime he is about to commit,
onto the cat, which looks very much like the victim of an earlier crime.
 

The second cat represents a number of things to the narrator: an earlier
mistreatment, an earlier victim, and perhaps, through the patch of white, even the narrator's
future act for which he is arrested and (probably) sentenced to death.  All of these acts
represent the narrator's guilt, even if, in true Poe fashion, he is unable to recognize and
accept it for himself.