Professor Thaddeus J. Pinchworm is the main antagonist of the "Birthday World" episode of The Wacky Adventures of Ronald McDonald. He is a professor and the owner of the episode's titular corrupt amusement park, whose goal is to use a ray to turn anyone it zaps into toddlers so he can rule the world.
After overhearing Hamburglar's lamentations about having nothing to give to Ronald McDonald for his birthday, Pinchworm uses him as a way to take over the world and talks him into giving him free tickets to Birthday World. Later, Hamburglar takes Ronald and the rest of the gang to the park and gives Pinchworm the tickets, only to then find out the doors are locked. They try out certain rides, which don't turn out any good. Pinchworm then talks the gang into going on a roller coaster called "The Ride of Your Life", much to the gang's dismay. At the end, a ray zaps the gang, rejuvenating them into babies and toddlers. However, the gang escapes when Ronald, using a pogo stick, unlocks it. Pinchworm, finding out the gang has escaped, summons his robot dog Cogsta (however it was spelled) to find Ronald McDonald, describing him. However, after being tricked into fetching a bone (made from a piece of metal junk) by Ronald himself, despite finding him, he brings it to Pinchworm, much to his anger. Because of this, Pinchworm traps the gang, and threatens them with no dessert after dinner if they don't stay in one place. Ronald, using pepper Grimace lends him, causes a cloud to rain a hedge maze, thinking it should slow him down. He does, however, find them by breaking through one of the hedges and interrogatively threatens them with skipping a bedtime story. For this, Ronald whistles a hedge sculpture and gets him to use Pinchworm for a toy.
Next, Hamburglar points out a barn with a sign (looking like Pinchworm) telling them "Ronald and Friends Hide Here", only for it to turn out to be a trap, where Pinchworm reveals his plan: to turn the entire population into "wee little children" with his ray, so he will rule the world. Soon after, before it is time to throw the big switch, all except Hamburglar are trapped, so he uses one of Ronald's birthday gifts (which looks like a small model in which his head pops out with a spring), and Hamburglar saves himself and his friends, turning them back to normal, and Pinchworm into a baby. Finally, Pinchworm meets the final part of his defeat when his robo-dog chases after him as "food".
He was voiced by Phil Snyder.