Meekrob is the planet Invader Tenn was assigned to. The dominant species native to the planet are highly intelligent beings of pure energy, which would make it hard 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, but instead received a package of malfunctioning SIR Units, which were meant for Zim; Smikka Smikka Smoodoo of the Conveyor Belt Planet switched the labels on two packages, so Zim ended up getting the Megadoomer.
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, and while this is probably how they look in real life, Zim's track record of getting his facts wrong provides an element of doubt.
It is also unknown how the planet Meekrob looks like from space.