The Koos is Loose is the second part of the 4th episode in Season Two of Dexter's Laboratory. It aired on August 6, 1997. In this episode, Dee Dee leaves Koosy in Dexter's care as she's at ballet class. However, Koosy's antics soon start to annoy Dexter and he sends him away.
Dexter is working in his laboratory when the door opens up behind him and a bright light making a laughing noise comes out and goes up behind him. He turns around to see what it is and the light knocks him out cold. Dexter wakes up to see Dee Dee standing there and he immediately starts accusing her of being the one who knocked him out. Dee Dee states that she did not do so and that it was really her imaginary friend, Koosalagoopagoop. Dexter does not believe in her and tells her that he is imaginary and does not exist . Dee Dee argues back and says that he is a real living animal and that she is the only one who can see him because she is the only one who truly believes in him. Dexter goes through his computer and looks up Koosalagoopagoop to prove to her that it does not exist and just as he expected, he does not. Dee Dee asks why he doesn't believe in him and he says that it is because he cannot see him. Dee Dee argues back that he can't see air either but he knows that, that exists. Dee Dee had officially convinced Dexter that he exists and all of a sudden, Koosalagoopagoop magically appeared in front of him and started laughing. Dexter, shocked to see this, stands and stared in awe for a while, and then he pulls on his tail. Koosalagoopagoop picks him up off of the ground and Dexter asks about him. Koosalagoopagoop introduces himself and tells about himself. He says that he comes from Dee Dee's imagination and that he can also be addressed as "Koosy" for short. Dexter then tells him to get out of his laboratory but Dee Dee tells him that he can't because he is too big to fit through the door, then before Dexter can respond, Dee Dee realizes that she is really late for her dance class and she runs off leaving Koosy there with Dexter. Koosy just sits there looking at Dexter and Dexter stands there angrily looking back at him for a while until Koosy suddenly breaks out mentioning that if you turn your lips inside out, that you would look like Jimmy Carter.