District 9's crustacean aliens may be the first extraterrestrials to experience South Africa's apartheid, but they're hardly the first species to feel the sting of oppression. We list science fiction's other downtrodden, enslaved, and dehumanized (so to speak) species.
The Newcomers (Alien Nation): District 9's aliens are most often compared to the Tenctonese, better known as the Newcomers. Like the D9 aliens, the Newcomers just can't catch a break. After fleeing from slavery on their own planet, a quarter of a million Newcomers land in Los Angeles to find a sometimes less than welcoming human population. Aside from the unfortunate names some INS officials assign the new arrivals (in the original movie, Matt Sykes' partner was named "Sam Francisco"), there are anti-alien Purists who think the Tenctonese should have stayed on Tencton, and plenty of murder, both from humans looking to eradicated the Newcomers and from those who would harvest their life-extending glands.
The Citizens of the Dominion (Star Trek: Deep Space Nine): With all of its explorations of race and morality, the Star Trek universe has had its fair share of oppressed species: the Troglyte miners who served their fellow Ardanans, the Romulans' Reman slaves, the Orion women (who only appear to be slaves), the Tosk who serve as prey for the Hunters' sport, the Bajorans who endure 50 years of Cardassian occupation, and, of course, anyone who encounters the Borg. But the Founders of the Dominion have a special talent for oppression, from engineering the supersoldier Jem'Hadar with an innate addiction to the drug ketracel white to infecting the Trevean with a congenital blight. Even the Vorta, who serve as the Dominion's middle managers, are mere slaves to the Founders, and are compelled to commit suicide if it serves their masters' purposes.
Clone Troopers (Star Wars): Slavery runs rampant in the Galactic Empire, with the Empire itself enslaving species like the Wookiees and the Mon Calamari wholesale, and some races, like the Twi'leks, would sell their own children into slavery in hopes of offering them a better life. And biological species buy and sell sentient droids (and ban them from their bars) without a second thought. But the genetically engineered (though otherwise human) Clone Troopers hold a special place among Star Wars' oppressed beings. Not only are they specifically grown for compulsory military service, they are essentially the property of the Galactic Republic, a government that has supposedly outlawed slavery.
The Ood (Doctor Who): Humans looking to have their own sentient slave without the guilt were told they could pick up an Ood servant with minimal damage to the conscience. After all, the Ood live to serve, right? Nothing in the Doctor's universe is ever so easy, and Donna and the Doctor soon discover that Ood Operations, the company supplying the alien servants, had cut off the Ood's telepathic link to the Ood brain, hampering their free will and leaving them to mix drinks and do laundry for their human masters.
Banik (Farscape): Oppression is a fairly widespread characteristic of the Farscape universe. Pretty much anyone living under Peacekeeper rule has a few humanoid rights trampled on (including the Peacekeepers themselves), and Scarrans have a pair of servant races who provide them with soldiers, intelligence agents, and technicians. But the Baniks hold an especially low place in the Farscape hierarchy. Having been mostly wiped out by Peacekeeper forces, the remaining Baniks have been enslaved, and the Banik Stark is repeatedly subjected to Scorpius' Aurora Chair, a torture and interrogation device. But the casual disregard for the lives of Baniks reaches its most shocking low when Scorpius purchases a lot of slaves that includes 9,999 Baniks and D'Argo's son Jothee. After he purchases the lot, Scorpius hands the slaves over to Natira, who, having no use for them, simply exterminates them all.
Sewer Mutants (Futurama): The 31st Century has little respect for humanoid or alien life, but at least most life forms are afforded the common courtesy of being able to walk the Earth's surface. Sewer mutants have no such privileges, requiring special permission to leave the subterranean ruins of New York. Sewer mutants, in turn, stick it to the sub-mutants, who are relegated to the sub-sewer (probably New York's original sewer system).
House Elves (Harry Potter): House Elves are powerful magical beings, with the ability to repel some of the most powerful wizards to come out of Hogwarts. But most of their magic goes toward serving their often less than noble wizard masters. House Elves are compelled to punish themselves if they disobey their masters or even utter an unkind word against them, and at least one ancient wizarding family held onto a gruesome tradition of decapitating elderly House Elves, then mounting their stuffed heads on the wall.
Dracs (Enemy Mine): Humans and Dracs are in the midst of a bitter war, so it's little surprise that the humans tolerate scavengers who capture Dracs for slave labor. But it also helps a brutal set of outlaws thrive without concern for human laws or Drac life.
Denizens of the Kzinti Empire (Known Space): The Kzinti began their lives in the galactic community as mercenaries, but once their Jotok clients taught them to use their weapons and technology, the Kzinti quickly turned on them, enslaving their former employers. From there, the Kzinti spread out across the galaxy, enslaving or eating any species they encountered. Although some subject worlds were more or less ignored by the Kzinti, some species were pushed off their worlds entirely, and breaking Kzinti law meant execution by hunting (usually followed by a feast featuring the accused as the main course). Even Kzin females, termed Kzinretti, are oppressed by their males, having been rendered subsapient by the hijacked Jotok technology.
Vortigaunt (Half-Life): Vortigaunts are the slaves of slaves, used by the Nihilanth as military forces or as factory workers. Although their enslavement forces the Vortigaunt to oppose Gordon Freeman in the first game, they get a bit of a happy ending when Freeman kills the Nihilanth. Once freed of their extradimensional masters, the Vortigaunts seek to keep humanity from falling to a similar fate, working against the Combine forces.
Neosapiens (Exosquad): Artificially created for life as laborers on Mars, the Neosapiens are stronger and faster than Terran humans, require little food and no sleep, and have a longer natural lifespan than their masters. You would think humans might think twice before creating such a physically advanced race only to enslave them, but they have to deal with the consequences in the ensuing rebellions. But the Neosapiens were not above creating servants of their own, engineering the animalistic Neo Warriors to serve as the Neosapiens' ground forces.
Mutants (X-Men): "Have you tried not being a mutant?" The classic line from X2 pretty much says it all. It's bad enough that the mutants have to cope with powers they don't always fully understand, or that their lives are punctuated by the occasional supervillain attack or alien invasion; they also have to cope with humans who hate and fear them, and religious fanatics who see them as an affront to God.
Cylons (Battlestar Galactica): Artificial beings have been oppressed by humans since Karel Čapek's R.U.R. premiered, and they've been turning on their masters just as long. The Cylons get bonus points, not because the nature of their oppression is unique, but because they're simultaneously portrayed as essentially human and yet dehumanized by their human enemies. Even forgetting racial slurs like "toaster" and "skin job" used to remind humans that their fleshier foes still have robot parts, there are some in the Colonial Fleet, like the rapist members of the Pegasus crew, who are inclined to treat the humanoid Cylons as warm-bodied objects. And the Cylons continue the cycle of oppression, with the humanoid Cylons effectively lobotomizing the Centurions and treating the Raiders as glorified pets.
Humans: Humans are the oppressed species nearly as often as they are the oppressors. Sometimes, we're enslaved by our own creations, as in the Matrix trilogy. Sometimes we've simply lost out as the dominant species of the planet, as in Planet of the Apes. Sometimes an alien invader simply decides we'll make good slaves, as in Stargate or Battlefield Earth. But we're a reliably plucky species, and even if we don't manage to pull ourselves out of the gutter, we don't make life easy for our oppressors.