Highly intelligent beings of pure energy
Meekrob is the planet to which Invader Tenn was assigned. The dominant species native to the planet are highly intelligent beings of pure energy, which would make it difficult for Tenn and her SIR Unit to disguise themselves.
The Meekrob are sworn enemies of the Irken Empire. In the episode "Megadoomer", Tenn was supposed to receive a Megadoomer Combat Stealth Mech as a gift from her leaders, but - thanks to a rebellious worker on the Conveyor Belt Planet switching the labels - instead received a package of deadly malfunctioning SIR Units, which were meant for Zim.
As a result, Zim ended up receiving the Megadoomer, while Tenn got the psychotic robots, which promptly began to destroy her base and ruin her cover.
The Meekrob made their first and only physical appearance in "Dib's Wonderful Life of Doom". However, these were not real Meekrob, but illusions created by Zim. While this illusion may be accurate to their true appearance, Zim's track record with factual accuracy provides an element of doubt. It is also unknown how the planet Meekrob looks like from space, though the terrain appears to be rocky and barren.