Martian

Martian
Homeworld	Mars
Skin	Unknown
Eyes	Unknown
Notable Facts	-Extinct -Huge Heads -Extremely Intelligent
[Source]

Martians were a species that were native to the planet Mars. They were an extremely advanced species, as evidenced by their apparent project of transforming their homeworld into a huge spaceship after testing the technology on a nearby planet. Ironically, they worked themselves into extinction doing this, so there are no more Martians living on Mars. The only "Martian" left is the Mars-ship's interactive, holographic instruction manual, which appeared in "Battle of the Planets".

The Martians' most noticeable feature was their enormous head; their eyes, nose and mouth were compacted together on their lower head, while their forehead dominated the other 3/4 of the cranium, suggesting that their brains were unusually large. Despite this, they didn't think of the consequences of their actions, as they only thought that turning the planet into a vessel was "cool."

Unlike most descriptions of Martians in pop culture, the "Martian" Zim encountered seemed overly friendly, so it's possible that the Martian civilization was not hostile and they were once peaceful towards their untimely end.

Trivia

• Zim was unaware that humans refer to inhabitants of the planet Mars as Martians, so he coined the term "Mars-oids" to refer to them.
• Their appearance from the neck down is unknown, as only skulls were ever seen, which suggests that the Martians may not have had actual bodies, and Zim's description of "floating heads" may not be entirely inaccurate.
• The Martians have a slight resemblance to Yummy Nuggets, as they're both species without bodies.
• The only piece of Martian technology ever seen was the planet itself.
• It can be assumed that because Mars is a desolate planet, the Martians are responsible for how it became due to working on turning the planet into a space ship.
• The Martians also built huge pyramids similar to those in Egypt, as part of their spaceship. However, most do not appear to be part of the spacecraft, and were probably some other form of architecture in days long past. These may also suggest that the Martians could have visited Earth many years ago, as homage to the paranormal theory that aliens helped build the pyramids, but this is never mentioned on the show.