Once you unlock the Pet Cat souvineir, you can make lightning appear around it's head, make it sweat and make it breathe flames out of it's nose.
To make lightning appear around it's face, turn your gameboy left and right really fast and the cat will growl and scream and lightning will be around it's head. To make it sweat, do the same thing to make lightning, and after it screams, pet it on top of it's head and it will start to sweat.
To make flames come out of it's nose, do the same thing to make it sweat except pet it on the left or right sides of it's head. It may take a couple of tries to do.