The Knucklehead is a spider-like robot built by Baxter Stockman, and named by Shredder (after Rocksteady, who was used as a guinea pig). It first appeared in the 1987 series episode "Enter The Fly" as Shredder used it against the Turtles, but the Turtles quickly defeated it.
It would later appear in a larger form in the episode "Casey Jones: Outlaw Hero" where it is used by Krang. However, the Turtles were able to infest it with robotic insects that cause mechanical malfunctions, causing the giant Knucklehead to destroy itself.