Mutagen raygun

A mutagen raygun is given by the pandimensional spacecraft's AI-computer to in the  episode Bye, Bye, Fly.

Using it, Baxter Stockman mutates into a fly, and Michelangelo into a gerbil. Those who get zapped are transformed into the size ordinary animals, but keep their intelligence and ability to speak.

The other later manage to retrieve the mutagen raygun, and restore Michelangelo. While and  later get their hands on it, and escape with Shredder, they don't restore Shredder onscreen.