Irken

Irkens are on average small green humanoids with a highly unconventional, although successful and highly advanced civilization with aspirations for conquest. The Irken Empire has very little to nothing to do with the RBS, as they tend to stick to their Armada and the planets that they've conquered in their grand galactic conquest campaigns, Operation Impending Doom (Numbered as appropriate). They have very little desire for diplomacy and trading as they simply take what they want as the desire arises. Irken Invaders would be the most likely to be seen on their own, mingling with members of other races and species, but they very rarely leave the planets they've been assigned. Of course, there may, however, be one or two Irkens that have gone off of the reservation, so to speak.

http://zim.wikia.com/wiki/Irken

http://zim.wikia.com/wiki/Irken_Biology