For Navi, the left cat, we call her Chunka Munk, Plasticat, and Ms. Infected Lips.
For Scorpi, the cat on the right, we call him Mr. Small, Spider Cat, and Crybaby.
My favorite cat was named Buddy, but since he would get into a lot of mischief we would often refer to him as "the monster". Once you start calling your cat Buddy Monster, you'll find you're now a very short distance from Monster of Buddies I'm pulling your strings.