Some TMNT stuff was always meant for grownups.
King Komodo is a mutant komodo dragon, who refers to himself as Go-Komodo's "half brother". At one time, he was just one of Go-Komodo's pets, but he was accidentally injected with a synthetic mutagen during the Turtles' rescue attempt of Splinter, but in fact, he had already been purposely injected prior to that with mutagen mixed with Go-Komodo's DNA. Believed to have been killed by Donatello when he blew up Go-Komodo's mansion, King Komodo was instead rescued and trained by Pimiko.
In TMNT #17, he killed two children, which caused the local government to warn of alligator attacks. Fearing that Leatherhead had turned completely feral and was behind the attacks, Leonardo went down into Leatherhead's abode to investigate. Instead, he found Leatherhead tied up and gagged, and that it was all a trap set up by King Komodo. Leo fought Komodo and his non-mutated brethren, and during the fight, he bit off Leo's left hand. Leo subsequently passed out from the pain.
Later, Leonardo awakens to find his wound bandaged, and King Komodo kneeling near him. Komodo expositions his story to Leonardo, and the Turtle goes to free Leatherhead. However, King Komodo has the ability to dominate minds more primitive than his own, which unfortunately includes Leo's now-free friend. Leonardo and Leatherhead battle, and Leo cuts off Leatherhead's oxygen underwater. When Leo resurfaces, he finds that King Komodo is pointing a rifle straight at him. Fortunately for him, Michelangelo and Casey Jones arrive in time to knock King Komodo into the water.
King Komodo was retrieved by his brother and instructed to attack Pimiko, whose goals now were at odds with Go-Komodo's. However, King Komodo had had enough time to adapt to his "brother"'s DNA and the mutagen in his system, forming a higher level of sentience than before and a mind of his own. Siding with Pimiko since she saved him and nursed him back to health while Go-Komodo ran in cowardice, leaving him behind to die, he uses his ability to control "lesser" life forms to subdue Go-Komodo and regress him to his komodo dragon form. Pimiko then slaughters him.
With his newfound ability to transform into a human-like visage similar to his brother's, he takes over Komodo Industries. He ensures that once Karai is done healing, she will be released, and apologizes for eating Leonardo's hand. King Komodo debates whether to restart the Foot Clan with Pimiko or just run Komodo Industries and make loads of cash, but either way, he is no longer an enemy of the Turtles and bids them farewell.