The scientist Dr. Baxter Stockman took a number of Demodogs and merged them with advanced technology, converting them into cyborgs. These creatures were Stockman's next generation of Mousers; they had fleshy yet durable bodies, and a metal endoskeleton.
When the Demodogs tried to attack the Party in the New York City subway, the Ninja Turtles fought them off, though they had no idea what the creatures were.[1]