Star Trek made its name by having Starfleet seek out new life. However, not all of that new life was friendly. During their many space explorations, the Federation ran into some scary alien races. Whether they were humanoid, reptilian or aquatic, some of these alien races turned out to be downright deadly.
But besides being deadly, what made these alien races introduced in Star Trek scary? Members of these notorious species could strike fear into the heart of the most experienced Starfleet officer. With eight live action and three animated Star Trek series from which to choose, these are the scariest alien races.
10 Cardassians
The big bads of Deep Space Nine, the Cardassians are a race of high-minded and cultured individuals. However, a need to survive forced them to become brutal and materialistic. The Cardassians are thought to be more dangerous than the Romulans, and the Dominion War proves it.
The Cardassians are clever, crafty and scary. They are prejudiced and totalitarian especially in their quests to conquer Bajor. While not all Cardassians are frightening, as a race, their actions reverberate through the franchise.
9 Romulans
The Romulans were presented as the main antagonist of The Original Series, much like the Borg or Cardassians would be in later series. Appearing in the first season of the original Star Trek back in 1966, the Romulans have been a thorn in the side of almost every captain in the franchise.
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A cousin to the Vulcans, the Romulans chose the exact opposite path of the species that loves logic. The Romulans are short-tempered, wrathful and militaristic. Their thirst for conquering every opponent in their way is frightening, and their dedication to secrecy makes them a hard enemy to strategize against.
8 Klingons
These may be the most famous Star Trek villains of all time. The Klingons are a warrior race whose empire was often at odds with Starfleet. Even though tensions cooled, and the Klingons would eventually join the Federation, the Klingon Empire remains antagonistic throughout a large part of the Star Trek timeline.
Depending on when the Klingons are presented, they are visually frightening. During The Next Generation, Klingons had ridged foreheads and looked kind of like lions. In other times, their foreheads weren't as ridged, but their lust for battle and blood make them just as scary. Even when they're allies of the Federation, they are not to be crossed.
7 Jem'Hadar
The Jem'Hadar are a race of genetically engineered warriors used by the Dominion in Deep Space Nine. The Jem'Hadar's growth cycle is accelerated, allowing them to go from birth to warrior on the front line in very little time.
The Jem'Hadar possess incredible strength and are fierce combatants who adhere to strict rituals. This race is scary because they have on interest in comfort or personal liberty, only combat.
6 Remans
What happens when the Romulans turn into even bigger monsters? The result is the Remans, a distant cousin to the Vulcans and Romulans. The Remans are basically the monsters that Romulans see in their nightmares.
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Some Romulans hire Remans as bodyguards, because Remans are believed to be better warriors than Romulans. Many Remans are designed to look like vampires, making them visually frightening on top of their menacing presence.
5 Borg
The Borg are the most recognizable villain of 1990s Star Trek. The villains instantly became iconic when they made their debut in The Next Generation. The Borg's quest for assimilation and their success in assimilating Captain Picard as Locutus is one of the most unsettling attacks in Star Trek.
The Borg became such iconic and still scary villains that they were brought in as a villain in the film, Star Trek: First Contact, as well as appearances in Voyager and Picard. These robotic creatures are some of the most horrifying in the franchise.
4 Species 8472
This is the only alien race that has ever struck fear into the heart of the Borg. That fear was so palpable the Borg struck a temporary alliance with Starfleet to combat Species 8472. These creatures have a genetic makeup that makes it impossible for the Borg to assimilate them.
The biotech for Species 8472 stretches far beyond the imaginations of the Borg or Starfleet. It's not clear why this species is so destructive, but any species that can scare the Borg and is hell-bent on destruction is truly terrifying.
3 Salt Creatures
These are some of the ugliest and grossest aliens introduced in The Original Series. While its official name is M-113, the aliens have been dubbed the Salt Creatures, because they remove the salt from its victims' bodies.
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It's essentially a space vampire that uses suction cups to feed on its prey. Salt Creatures possess a strong grip and an oblong mouth that can send chills down the spine. Visually, they look like the stuff of nightmares.
2 Q
While irritating for Captain Picard, Q remains one of the most popular aliens in the history of the franchise. Q is a member an unknown race that is able to travel through time and toys with Starfleet crews. He appears as a humanoid when addressing Starfleet, but his true form remains unknown.
Q's motives are often unclear, but it seems like he is looking for a true nemesis he can respect from Starfleet, especially in Captain Picard. Q is one of the most powerful beings in Star Trek and one that can alter the fabric of reality, making him and his race very frightening.
1 Vidiians
The Vidiians represent the worst of all alien races. They are basically organ harvesters. Afflicted with a disease that eats away at their body and organs, the Vidiians butcher other alien races to use the other aliens' skin and internal organs to stave off death.
The Vidiians are cold and unrelenting. They justify their actions about having the right to live, while impeding on that same right for others. In an episode of Voyager, a Vidiian separates B'Elanna Torres into a human and a Klingon, and then tries to persuade the Klingon to be with him by killing a member of the Voyager crew.