Remember the classic Star Trek episode, "Mirror, Mirror", which was later adapted into a comic book? Well, that was the inspiration, one could guess, for the World's Greatest Super Friends episode, "Universe of Evil". Conveinently, the Wonder Twins had gone on vacation at the start of this adventure. Their evil counterparts make a cameo appearance early, but are not heard from again.
Edit, 7/26/18: Dailymotion deleted the video. Soon as we find another copy, we'll bring it back.
Too bad this couldn't be a 2-part episode.