Yaap Yaap was a creature who grew from a pimple on Billy's back after he drank Granny Grim's potion to heal his stomachache after eating too much junk food. He only appeared in the episode Billy's Growth Spurt.
He originally started out as a pair of eyeballs and a mouth, until Billy fed him and he grew more and more. With each growth spurt, he grew to resemble Billy more and more until he was strong enough to detach from him. He resembled Billy in appearance except he was very small and naked.
Billy called him Yaap Yaap because that was all he could say. Mandy and Grim thought of a plan to get rid of him as he is wrecking Billy's kitchen. Grim receives a potion from his grandma to kill him. When Yaap Yaap sees this he runs under the couch, and when Grim goes to retrieve him he bites him in the butt. Mandy tells Yaap Yaap that the potion is actually soda to which he excitedly gulps it all down. He then lets out a loud belch and melts away to a puddle of flesh. Billy is heartbroken because he is dead, and that he wanted the soda. Mandy sucks up Yaap Yaap's remains in a little vacuum cleaner and tells Billy to throw them away, and Billy considers it, but eats it instead and yells "Yop Yop!".
- He, like Billy, was voiced by Richard Steven Horvitz.