The man's choice of weapons in the murder of Pluto strikes us as rather odd. The image of a cat hanging from a tree in the garden all day, and even at night when the man goes to sleep is profoundly disturbing. When the narrator notices that the second cat has an image "of THE GALLOWS!" on it's fur, we might feel a tad bewildered. A gallows is a structure used to hang people.
The second cat wears a symbol of Pluto's murder on its body. It becomes a symbol of the man's guilt and depravity, a visual reminder of his crime, and of his changing personality. It also foreshadows the man's own death by hanging. (Though we don't know for sure if this sentence was carried out.)
Now, Poe is often considered a southern writer. He spent much of his life in the South. In his day, before abolition of US slavery, it wasn't necessarily uncommon to see a black person hanging from a tree. It seems doubtful that Poe didn't have this in mind when he wrote the story. In Romancing the Shadow: Poe and Race this possibility is discussed at length, and we look at it more closely in Pluto's "Character Analysis."