Go to Spy Island. When you get out of the blimp, go left until you see a little black cat. Click the cat, then quickly open your menu and go to your friends. Click a random friend then let your friends walk until you can't see the selected friend anymore. Press the "customize" button. The Poptropican will look like the cat.
Take the whiskers and the ears from the cat. That is all you can get from the cat.
Go to a common room and take a black shirt and pants from someone. You will also be able to get the tail from the common room. If you have active membership, you can get it from the Monster Carnival Island costumes.
Finally, go to the Home Island and go into the "New You" shop. Go to the customizer, and make your skin black.
You're finished! This costs 0 credits.
If you can't find everything I stated here, add this username. 2002tabikinz or 2002tabikinz1.