What mark did the second cat have on his chest?

At first, the mark on the cat is only a white patch. Later, however, the narrator thinks it has changed shape to appear like the gallows.

The change in the cat's marking likely foreshadows the narrator's fate. At the beginning of the tale, he says it is the last night of his life because he is being put to death the next day for murdering his wife. A gallows is a construct where a person...

At first, the mark on the cat is only a white patch. Later, however, the narrator thinks it has changed shape to appear like the gallows.

The change in the cat's marking likely foreshadows the narrator's fate. At the beginning of the tale, he says it is the last night of his life because he is being put to death the next day for murdering his wife. A gallows is a construct where a person is hanged. It usually includes a platform, two upright posts, and a beam connecting them from which the person is hanged. 

The cat with the mark of the gallows is what ultimately leads the police to find the narrator's murdered wife. When the police are preparing to leave the basement where she is buried, the cat meows in response to the man knocking on the wall. When the police tear down the wall, they find the body of his murdered wife with the cat on top of her. The narrator is taken into custody and ultimately sentenced to death.