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].
In the 2003 series, Foot Mystics are a special team of Foot Clan members that can each control one of the five elements (fire, water, earth, metal, and wind). They first appeared in the episode "Return to New York, part 2" (October 2003), and stuck around for the rest of the series, even being the primary antagonists of The Lost Episodes, aka season 5a. They have two forms - the ones above and the ones below:
Foot Mystics later showed up in Tales of the TMNT vol. 2. Their first appearance was issue #2 (March 2004), in a script that was originally written to be part of the 2003 series tie-in comic. Despite that being the case, Foot Mystics became a recurring part of this series, although they seem - from my limited knowledge - to be more of a class of Foot Clan member than a specific group in this continuity. They also don't have the same design as the Mystics of the animated series...
...although that changes in Tales of the TMNT #43 (January 2008):