The Catwoman from Channel Six - S2-E12
Corrected entry: April is transformed into a Catwoman by a teleportation accident. Shredder puts a mind control collar on her, and she attacks the turtles. The turtles manage to remove the collar, and the fight ends. Shortly afterwards, April returns to her normal form, without any explanation.
Correction: Shredder himself provides the explanation within the episode, after he discerned April "transmutated with a cat." When the collar is put on April, Bebop asks Shredder why he's giving her such a pretty present. Shredder replies, "It's not a present, you cretin. The effects of the molecular crossover will only last a short time, but with that collar, she will obey my every command forever." So, while April is temporarily a cat, Shredder uses it to his advantage and controls her thought waves to go after the rat, Master Splinter, but alas, Shredder's control was terminated. Since the effects of the molecular crossover were merely temporary, April later returns to her normal human self.
Super Grover ★