They appear in the episode "A Thing About Rats". The Mousers were created by scientist Baxter Stockman to rid the town of its rat problem. The Mousers attacked Splinter, but he held them off until the turtles came to his rescue and destroyed the remaining ones. Shredder angrily ordered Stockman to program them to also attack turtles. The Mousers attacked Splinter and April at April's apartment, and again were held off until the turtles arrived to destroy the remaining ones. However, by this time, the Mousers' constant chomping had weakened the apartment building, and it collapsed, although everyone inside escaped in time.
Realising that Stockman was responsible, the turtles captured him for information. Meanwhile, Michaelangelo snuck into Stockman's mansion to shutdown the Mousers, but Michaelangelo was captured by Shredder, who then ordered the rest of the Mousers to attack the turtles, Splinter, and April. Krang freed Michelangelo because he didn't want Shredder to succeed, so Michelangelo confronted Shredder and taunted him, causing Shredder's anger to get the better of him. Shredder took out a gun and accidentally hit the master control, allowing Donatello to take control of the Mousers with a remote control he had swiped from Stockman. Donatello commanded the Mousers to bring down Baxter Stockman's mansion, which collapsed on top of them, destroying them.
Description and abilities
Standard Mouser robots are of the approximate size and shape of a turkey. They have a single eye and two enormous jaws with hardened steel teeth, which can easily bite through concrete, brick and metal. In addition, the Mousers are able to climb walls with their gripping toes, and have a cavity in their bodies that can be used to transport swallowed material or objects.
Standard mousers have little intelligence and no independent thinking; they are usually controlled from a central computer station by means of radio waves.