Despite being a minor character in the Mirage comics at the time of its initial airing, the Rat King (voiced by Townsend Coleman) is featured as a recurring character in the 1987 Teenage Mutant Ninja Turtles animated series, being one of the few villains from the comics to make the transition into the cartoon (the others being The Shredder, the Foot Clan, the Triceratons, and Dr. Baxter Stockman).
The cartoon counterpart of the Rat King was somewhat inconsistent in some regards to his comic version, being shown with blonde or orange hair instead of black and having a slightly altered costume; his first few appearances on the show had him controlling rats with a flute (à la The Pied Piper of Hamelin) instead of his mind as in later episodes. Even Splinter was affected by the music and almost killed the TMNT in a fight. The cartoon version of the Rat King was also depicted as highly intelligent, shown to be able to create such things as various chemical concoctions and bombs.
In the series, the Rat King is shown as a homeless man living in a dilapidated portion of the New York City sewer system near the Turtles and Splinter. In episodes featuring him, the Rat King would often enact some sort of plot to establish his own rat-controlled government and bring human rule to an end, believing that rats (which he counted himself as) were superior to all other species, whom he described as "inferior non-rodents." He occasionally joined other villains in group efforts that would, he thought, ultimately help his own goals.
In some episodes of the show, the Rat King is depicted as somewhat of an anti-hero, with him, on one occasion, even helping the Turtles rescue a captured April O'Neil. When not actively seeking to expand his "empire", the Rat King eventually seemed content to simply remain underground with his loyal rats and, it seems, realized it was more to his advantage to have the Turtles as allies than as enemies. His final appearance is in early season 8, with new attire, and mutated rats with the help of Shredder, however, the Turtles manage to capture him and turn him in.
In his final appearance in the series, he had added a tattered dark trench coat and dark hat to his ensemble and was recruited by a desperate Shredder to get rid of the Turtles in return for mutating his rat servants to help in his own schemes. As a clause to his rats being mutated, the Rat King was asked to help Krang retrieve a weapon of his from Dimension X that ended up in the wrong part of the city and was now being studied. He turned on Shredder and Krang, claiming the device for himself, planning to use it to blow up a reserve of rat poison for an ironic annihilation of the city's population. However, he was stopped by the Turtles and arrested.