Captain Kroolik is an alien circus owner (who appeared in the 1987 series episode "Back to the Egg") who travels the galaxy along with his freak circus of intergalactic mutants, mainly assisted by a mutant named Grillox, who has uncanny senses. Krang invented an age-reversing serum to turn the Teenage Mutant Ninja Turtles into toddlers and hired Captain Kroolik to help Shredder use the serum on the turtles. While Krang and Shredder would easily take over the city with the turtles too young to stop them, Kroolik was promised to keep the Turtles as new additions for his circus. The Shredder did not like Kroolik, mainly because Krang trusted him better.
After suffering humiliation at the hands of the de-aged Leonardo and Michelangelo, Captain Kroolik was last seen being turned on by his own mutant circus freaks, who took over his ship while keeping him prisoner, the cruel ringmaster pleading to be freed and insisting that he likes mutants as they flew off.