That's right: From June 3 to July 2, there'll be a post each day highlighting Teenage Mutant Ninja Turtles canon immigrants. Now, because TMNT canon is a little confusing, it can be hard to tell what counts as a canon immigrant and what doesn't. So for the purposes of this blog, a TMNT canon immigrant is anything (a) from the movies, cartoons, video games, toy lines, or tie-in comic series that later appeared in (b) TMNT volumes 1, 2, or 4 [because they're by the original creators] or TMNT volume 5 [because the original creators are involved, it's the new official continuity, and its goal is to use as much of the diverse TMNT lore as possible].
Metalhead is a robot turtle created by Krang and programmed with all the memories of the Turtles. He first appeared as an action figure in 1989, before appearing in the 80s cartoon in November of that year in the episode "The Making of Metalhead". Eventually, Donatello reprogrammed him to be a good guy.
In IDW continuity, Metalhead was created as a security robot by Donatello's ally and StockGen employee Harold Lillja, but a glitch caused him to target StockGen employees. He first appeared in Teenage Mutant Ninja Turtles #34 (June 2014) and was named by Angel.