They are native to the planet of the same name, which Irken Invader Tenn was assigned to conquer as part of Operation Impending Doom II. However, Tenn's base was destroyed because of a package of malfunctioning SIR Units, which were supposed to go to Zim. In a two-part season finale (whcich was never made due to the show's cancellation), Zim and Invader Skoodge would have gone to Meekrob and rescue Tenn after she was discovered by the Meekrob and held captive.
They are highly intelligent and can change their form at will, they look vaguely like floating sets of lungs with tentacles. The Meekrob are in alliance with The Resisty, whcich would have been revealed in the unmade season finale mentioned above.
When they appear to Dib they quickly transform into giant flying shoes, claiming that their real appearance is impossible for the human brain to comprehend. However, their true appearance is clearly visible before the transformation and Dib tells them he had no problem comprehending it.
- Invader Zim - "Dib's Wonderful Life of Doom"