The second black cat looks almost exactly like Pluto. He's big, black, and missing an eye. The only difference is the white spot. The spot starts off innocently enough, but then grows into an image of the gallows, if the narrator can be believed.
With all these similarities, and with the narrator's insistence that the cat is more than just a cat, we might think the second black cat is some kind of supernatural version of Pluto. How, we might ask, could the second black cat be missing an eye, if he isn't Pluto undead? There is a possibility that Pluto never died. But, the narrator tells us that Pluto was not only hanged, but left hanging all day and night, and then somehow embedded in the plaster wall thereafter. It's pretty doubtful Pluto survived. So what about the missing eye?
Well, if you think about it, in Poe's time there were probably plenty of stray cats with missing eyes. The second cat could have been abused by a previous owner. Or he could have lost it in a fight with another cat, or some other kind of accident.
We don't deny the possibility of the supernatural, but to focus too heavily on it might take the focus off the narrator's abuse of the creature. Interestingly, the man's own account of the cat seems to work against a supernatural possibility. This is the man's description of the cat's voice coming from inside the tomb:
[It was] at first muffled and broken, like the sobbing of a child, and then quickly swelling into one long, loud, and continuous scream, half of horror and half of triumph, such as might have arisen only out of hell […] (31)
If the cat was such a cunning monster, why would he 1) wait so long before crying out, and 2) cry like a baby when he did cry? If you were a poor animal, on the verge of release from being buried alive, wouldn't you be both horrified and triumphant? This terrible moment is effective in making us think of the cat as an innocent victim.