There are many races shown in the universe of Invader Zim.
The Races of Invader Zim
Irkens
The Irken Race is determined to eventually conquer the universe.
See also
Meekrob
The Meekrob are a race who oppose the Irkens; they are only seen in a hallucination created by Zim, so whether or not their appearance is accurate is to be determined.
The Meekrob definitely exist, though; Invader Tenn has been trying to conquer them in Megadoomer, and though it was never scripted, there was to be an episode revolving around them capturing her, with the Tallests sending all the other Invaders (save for Zim, though Skoodge informs him of the rescue mission anyway) to rescue her.
The Meekrob are beings of pure energy, which would make it hard for Invader Tenn to disguise herself.
The word Meekrob itself is an allusion; Meekrob is a Thai dish of crispy noodles, which Fillerbunny is forced to eat in Jhonen Vasquez's comics.
Humans
Humans are a typically disregarded species living on the "dirt ball" called Earth. In the semi-distant future of Invader Zim, humans are technologically more advanced, but far less intelligent than modern humans. As mentioned in Tak: The Hideous New Girl, most humans don't seem to notice anything suspicious.
Nhar-Gh'ok
The Nhar-Gh'ok are an alien race which look exactly like human babies (a similarity they were unaware of), but at times they take on a more menacing appearance (pictured right). They are quick and resourceful creatures, and strong enough to dent metal, so despite their appearance they shouldn't be taken lightly.
Among their other abilities, like vomiting acid and creating horrible smells to defend themselves, groups of Nhar-Gh'ok can combine into a single "Giganto-Baby". They were only seen in the episode Plague of Babies.
Planet Jackers
"Hey, look at the little green guy."
The Planet Jackers' homeworld orbits a dying sun, so they throw stolen planets into it like firewood to keep it burning. Zim saved the Earth from being stolen, because he was worried that when the Irken Armada got to Earth, it would not be there. They appeared only in Planet Jackers. They have a treaty with the Irkens, stating that all planets marked for conquest by the Irken Military are to be left untouched.
Plookesians
The Plookesians are a peaceful and intelligent species of travelers. The only two that have been seen, Mooshy and Spoopty, were lured to Earth by a false distress signal sent by Dib and Mr. Dwicky. They offered to give Mr. Dwicky a tour of the cosmos and gave Zim some superweapons, both of which caused no end of consternation for Dib.
It is never revealed in the series where the Plookesians are from, though it can be assumed that it is called Plookesia. One notable trait of theirs is that their arms are located above their heads, which are protected by helmets (although it seems that their limbs are mechanical in nature anyway). They made their only appearance in Vindicated!.
Screwheads
Smikka Smikka Smoodoo
The name of the race that Smikka Smikka Smoodoo belongs to was never revealed, though other Zim sites refer to them as "Screwheads", as there are huge screws in their heads. Their race has been conquered by the Irkens, and their planet converted into a galactic shipping hub.
Smikka himself seeks to inspire his fellow slaves to rebel, but so far the furthest they have gone is to switch labels on packages.
It is unclear if the Screwheads' eponymous screw is endemic to the species or if it was imposed by the Irkens. It bears resemblances to the brain probe Zim uses to monitor Nick's happiness center, and to the probe that Zim used to help control the emotions of his Santa suit.
They make their only appearance in Megadoomer.
Slaughtering Rat People
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The Slaughtering Rat People.
The stone structures that the Rat People inhabit.
The Slaughtering Rat People of Planet Blorch don't seem to be too technologically or culturally advanced. They are so horrible, in fact, the Almighty Tallest chose to give Blorch to Invader Skoodge as a sort of punishment. To their surprise and dismay, though, Skoodge actually managed to conquer the Rat People, and Blorch was turned into an Irken parking structure planet.
In their last appearance, the Rat People are scurrying about looking for cover as the Irken Armada engaged in their genocidal Organic sweep of the planet. They are now presumed to be extinct.
Vortians
Vortian Prisoner 777
The Vortians are an industrious and creative race, native to the planet Vort, home of the universe's most comfortable couch. Their engineers designed The Massive and much of the Irken military equipment. Planet Vort was conquered by Invader Larb during Operation Impending Doom II, despite the Vortians being allies of the Irken Empire. However, the Vortians may have been blamed for Zim's infinite energy absorbing blob, which he created when he was a scientist working on Vort Research Station 9. The blob devoured Tallest Miyuki, and then Tallest Spork when it came back to get its collar. Now there are pockets of Vortian resistance to the Empire. Lard Nar is one such intrepid Vortian, leader of the Resisty.
Unknown
Shloonktapooxis' Race
Shloonktapooxis is the member of an unknown race who resemble purple cones, with a single, curly antenna on their head. His species' homeworld has been conquered by the Irken Empire.
Spleenk's Race
Spleenk is the member of an unknown race, who are light brown in color, and have four arms. Spleenk also has a purple spot on his forehead; it is unknown if the rest of his species have this. His species' homeworld has been conquered by the Irken Empire.
Ixane's Race
Ixane is the member of a race who wear cloaks that cover their bodies. There were two of her species on the Resisty's ship, so she is not the only one angry about the Irkens. It is unknown what she looks like because of her cloak, although she appears humanoid.
Green Blob Species
One of the Green Blobs.
The Blob Species are a race of little green blobs, who apparently constantly vomit all over themselves. One of the Blobs, a female specimen with orange hair, is a member of the Resisty.They share several similarities to the mascot for Shloogorgh's Flavor Monster, and it can be assumed that this species' homeworld has been conquered by the Irken Empire.
Three-Headed Species
This species has three eyes, pinkish skin, and blonde hair. A member of this species is in the Resisty, and another one of his species, possibly the same person, worked with Lard Nar on the planet Vort. This species' homeworld has been conquered by the Irken Empire.
Bug-Eyed Species
This species is armless, and have bug-like eyes, green skin, and a horn on their chin. This species' homeworld has been conquered by the Irken Empire.
Insectoid
This species resemble Irkens, with thinner eyes and greyer skin. This species' homeworld has been conquered by the Irken Empire.
Headless Species
This species is very bulky, and have no heads, and instead a floating brain. It is unknown if they are capable of speech, as they have no visible mouth, as they lack heads. This species' homeworld has been conquered by the Irken Empire.
Green Species
This is one of the few species with human eyes. They have green skin, and appear constantly around the Resisty's ship. This species' homeworld has been conquered by the Irken Empire. They are also seen on planet Foodcourtia, once again in a pack.
Greys
This species have grey skin and huge heads. Their homeworld has been conquered by the Irken Empire, but it is unknown if they became slaves or were made allies.
Orange Blob Species
This species are fat blobs, with orange skin and one eye. It is unknown if their planet has been conquered. An example of this species is Eric the Blob.
Rock Species
These species are native to the planet Invader Flobee was sent to conquer. They are made of rock, or more likely, silicon. They are apparently quite stupid, as they fell for Flobee's hideous disguise, consisting of a few rocks strapped to his body. Their planet has presumably since been conquered by the Irken Empire.
The "Blob Thing" species
They are small, white blobs of an unknown planet. Only one of them is ever seen in the series in Abducted. He is the only specimen that the aliens had caught and did not escape. It is unknown if their planet has been conquered by the Irken Empire.
The Hobo species
They are native to the planet Hobo 13 and are allies of the Irken Empire. They refer to each other by using numbers. From what is seen of them, they are tough and militarialistic, which is perhaps the reason of their willingness to side with the Irkens.They somewhat look like a hybrid of human and irken.
Mantis species
They are brown-grey aliens that resemble the Greys, except they have arms and legs vaguely similar to a praying mantis. They are the aliens that abducted Zim in Abducted, but seem to be incredibly stupid, even more so than the humans. It is unknown if their homeworld has been conquered by the Irken empire, although it is unlikely, since they are not aware of the Irkens' existence. The Mantis species first made their appearance, not in Invader Zim, but in a short story in Jhonen Vasquez's other work, Johnny the Homicidal Maniac. In it they force a man to fornicate with a chicken and produce offspring.
Crystal species
They are humanoids that look like they are made up of pure crystal. A diamond-colored female one appears in Hobo 13, and a purple variant (probably male) is seen eating at Shloogorgh's in The Frycook What Came from All That Space. It is unknown if their homeworld has been conquered by the Irken empire.
Throbulator's species
Throbulator, Skoodge and the "Crystal Alien" on Hobo 13.
These are humanoids with huge heads "of pure headache". It is unknown if their homeworld has been conquered by the Irken empire.