Note the color of the brush while fighting the Painter in the Wizard's Castle. It will give a clue as to what he will paint next.

Light pink: Unicorn
Orange: Carrot Clock
Teal: Angry Person
Red: Big Monster
Purple: Octopus
Magenta: Clown
Turquoise: Zombie
Light orange: Cat
White: Elephant
Light purple: Nose-Snail