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].
Mondo Gecko first appeared in the 1990 toy line, along with other characters who would go on to become Mutanimals, such as Man Ray, Wingnut, and Mutagen Man.
The next year, he would appear in both the 80s cartoon and the comic tie-in, although these two versions don't match up. The Mondo Gecko that first appeared in Teenage Mutant Ninja Turtle Adventures #18 (February 1991) was a human mutated into a gecko, while the animated version that appeared in September of that year was a gecko turned into a humanoid.
Mondo Gecko 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.
In the world of IDW, Mondo Gecko appears as one of Old Hob's first successful mutations beginning in Teenage Mutant Ninja Turtles #38 (October 2014).