Trickster Superman villain Mr. Mxyzptlk will appear on two episodes of Supergirl Season 2.

Things are going to get really weird later this season on Supergirlwhen another Superman villain makes his way to the show. It isn't some kryptonite-powered powerhouse like Metallo this time, though. Instead, Comicbook.com reports that it's the 5th dimensional imp with reality warping powers, Mr. Mxyzptlk. Every now and then Mr. Mxyzptlk likes to show up in Metropolis to annoy Superman, because that's his favorite hobby, and the only way to get rid of him is to get him to say that difficult name of his backwards.
Now, for reference, the 5th dimensional imp's name is pronounced "mix-yez-pitel-ick." Accept no other pronunciations. As for what it sounds like when you get him to say his name backwards, Superman authority Mark Waid seems confident that it's "kil-tepz-yex-ehm" and that's good enough for me. This isn't to be confused with how his name was spelled in his earliest appearances, Mxyztplk, which I seem to remember reading somewhere is pronounced "mux-izt-pulk" but I could be wrong.
Mr. Mxyzptlk has been around since 1944, and he was created by Jerry Siegel (co-creator of Supes himself) and Ira Yarbrough. He's generally more of a trickster pain-in-the-ass than an actual villain most of the time. He pops in from his dimension Now, considering the nature of his powers, he's certainly capable of more, and in Alan Moore and Curt Swan's legendary "Whatever Happened to the Man of Tomorrow" story as well as Grant Morrison and Rags Morales' New 52 Superman reboot, we got to see a different side of the little guy.

This isn't his first appearance in live action, either. He appeared in all of his comic book glory on the SuperboyTV series in the '80s (which I swear I'm going to get around to writing about one of these days), played by a perfectly cast Michael J. Pollard, who delivered a remarkably disinterested performance. A less visual version of the character was played by Howie Mandel on Lois and Clark: The New Adventures of Superman, and he also appeared in unrecognizable form on an extraordinarily bad episode of Smallville.
Mxyzptlk was a staple of various Super Friends animated adventures, although his best moment outside of the comics remains his two appearances on Superman: The Animated Series, where he was voiced to perfection by Gilbert Gottfried. Both are essential viewing, and Mxy sports his 1940s look (although his modern spelling and pronunciation). Since Mxyzptlk is basically a Looney Tunes character in a superhero setting, those cartoons upped the Bugs Bunny/Daffy Duck factor, to great effect. Gottfried will return for the upcoming Justice League Action animated series, so that should be a treat.
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Mxyzptlk will apparently appear in two episodes of Supergirl. It's not yet clear if he'll have a previous history with Superman (note that Superman villain Metallo didn't for the purposes of this show). But if it's a two-part excuse to bring back Tyler Hoechlin as the Man of Steel, this could be a lot of fun, especially if it's an excuse to do some crazy, funny stuff with the show. Wait...what if this ends up being the reasoning behind the upcoming musical crossover between Supergirland The Flash? That would be insane.
I'll update this with more info as it becomes available.