The Black Top Hat is a hat accessory item in Nintendogs + Cats that was given out by Honest Abe from SpotPass DLC. It has a white counterpart available in the Downtown BARC. This item can also be found occasionally on the Secret Path in the 3DS versions. There is also a similar item, the Top Hat, available in Labrador & Friends.
3DS: "This hat is only for distinguished and discerning pets."