11. Species 8472 ("Star Trek: Voyager," 1995-2001)
Species 8472 had a cool combination of biology and technology that made even the Borg envious. For one thing, their immune system made them immune even to Borg assimilation. They also could hold off some of the more common transportation technologies in "Star Trek," such as transporters. Species 8472 also had cool bioships that shared some of their same genetic material, making for a pretty formidable foe in space. [Warp Drive & Transporters: How 'Star Trek' Technology Works]
12. Ocampa ("Star Trek: Voyager," 1995-2001)
This humanoid species had a life span of just nine years, meaning they had to pack in reproduction and the other experiences of adulthood into just a short time. But they made up for the short life span with a host of abilities humans don't possess, such as telepathy, telekinesis and the ability to sense matter at the atomic scale.
13. Xindi ("Star Trek: Enterprise," 2001-2005)
The Xindi were actually an alliance of six different species, of which five were still remaining in the timeline of "Enterprise" — aquatics, arboreals, insectoids, primates and reptilians. The sixth, the avians, were believed to be extinct due to a civil war between the species. None of the five species could agree on which was dominant. These species probably offer some kind of metaphor about how much genetic material humans share with other species on Earth, particularly primates.
14. Xyrillians ("Star Trek: Enterprise," 2001-2005)
We love the Xyrillians for their unique form of reproduction; females could impregnate males with a simple touch. In an interesting twist, it turns out that Xyrillians and humans are compatible, because a male crew member found out he was pregnant shortly after coming into contact (metaphorically and literally) with this species. After reproduction, Xyrillian males carried the babies to term using a pouch on their chest.
15. Unknown alien species ("Star Trek: Enterprise," 2001-2005)
This species freaks us completely out because we don't know who they are, and we can't clearly understand their intentions (although they do appear to be somewhat hostile). In the episode "Silent Enemy," the Enterprise sends messages saying they don't want to be hostile, but they will defend the ship; they eventually manage to scare the species off with an attack, and the unknown alien ship disappears at warp speed. According to the fan site Memory Alpha, "Star Trek" creators said they made this episode to demonstrate that in the early days of exploring space, there will be mysteries.