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].
I should say upfront that "Leatherhead", as a concept, is not a canon immigrant. He first appeared very early on in the Mirage comics and has appeared in most adaptations since. I'd say that, next to Slash, he's probably the most enduring non-Turtle clan mutant.
That being said, this Leatherhead is a little bit different. Instead of being a mutated alligator, he's a human named Jess Harley that was mutated into an alligator. He's also from Louisiana, which none of the other Leatherheads are. He first appeared in Teenage Mutant Ninja Turtles Adventures #6 (October 1989), and would later go on to join the Mutanimals.
Side note: despite this series being a spinoff of the animated series, the animated series actually has a different Leatherhead. Although the characters look very similar, that one did in fact start out as an alligator instead of a human, and is also from Florida instead of Louisiana.
Leatherhead would later go on to cameo with (most of) the rest of the Mutanimals in Mirage's Tales of the TMNT #58 (May 2009), although it's important to note that this is still the "Adventures" version because it's in a scene of the multiverse.