The short answer to this question is Bebop, but there is a lot of history behind this character to explore.
While Shredder was the most notorious villain in the TMNT series, there were also other villains including Shredder’s two henchmen, Bebop and Rocksteady. Both characters were originally human thugs that were employed by Shredder. Bebop was a man sporting a Mohawk while Rocksteady was a short, blonde, burly man. In terms of intelligence, both men were portrayed as simpletons.
Bebop and Rocksteady’s story of transformation into anamorphic characters begins when Shredder sends his street gang to stop news reporter, April O’neil, from doing a report on crime in the city, The Ninja Turtles come to April’s rescue and defeat Shredder’s gang in combat. In an effort to retaliate, Shredder turns two of his gang members into mutant creatures. Bebop and Rocksteady volunteer to undergo the procedure in order to exact revenge against the Turtles.
Shredder mutates Bebop into a warthog and Rocksteady into a rhinoceros. Both villains retain some distinct characteristics of their former human self. Bebop still had his purple Mohawk and punk gear, while Rocksteady maintained his soldier-look by sporting an Army belt and helmet, Although the two characters became physically larger and stronger, they still lacked intelligence and thus failed to defeat the Turtles in any battle. Their incompetence and goofy behavior overshadowed any advantage they gained from their physical transformation.
Interestingly enough, both Bebop and Rocksteady are names that were taken from music genres. Bebop is a style of Jazz while Rocksteady is a Jamaican style of music originating from the late 1960s.