Genghis is one of the Punk Frogs, a group of four frogs that were mutated at some point in the past.
In Holiday in Florida!, Krang tricks the Turtles into thinking they've won a free vacation in Florida. Once they are there, Bebop and Rocksteady lure them into the swamps and use the mutagen given to them by Krang to mutate four frogs into giant frogs. At first, the Turtles think they are seeing their buddies, the Punk Frogs, but quickly realize these frogs are much larger. The new frogs promptly attack the Turtles and disarm them, overwhelming them with their strength. Watching from Dimension X, Krang is delighted, believing he's outthought and overthrown the Turtles.
Fortunately, the real Punk Frogs arrive on an airboat and witness the battle. They hop into action and join the fight, with Genghis kicking one of the giants to the ground. This gives the Turtles time to find their weapons. Once more armed, the Turtles rejoin the battle and quickly defeat the giant frogs. With the giant frogs out cold, the Turtles and Punk Frogs turn their attention to Bebop and Rocksteady, who beat a hasty retreat.