
Salt Vampire
Episode: "The Man Trap"
Description: Looking like a cross between a lamprey and a troll doll, this creature needs salt to live. Rather than just hitting the Wendy's drive-through, it starts jumping Enterprise crewmen and sucking the salt from their bodies. It can change shape, or maybe just make people think it's changing shape. Either way, Bones totally had the hots for it.
Powers: Shape changing, salt sucking, standing up to Spock's hammer punch.
Weaknesses: Phasers, chastity.

Gorn Captain (My Avatar)
Episode: "Arena"
Description: A big lizard guy who sounds like a cross between a leaky steam pipe and a constipated pirate. He's strong and tough, but so slow-moving you'd think he worked for the DMV. Kirk fights him on the Asteroid of Unlikely Mineral Deposits, defeating him by inventing gunpowder and blasting him with a diamond the size of William Shatner's ego.
Powers: Strength, imperviousness to fake-looking Styrofoam rocks, flint knapping.
Weaknesses: A diamond to the gut, godlike aliens.

Horta
Episode: "Devil in the Dark"
Description: The single remaining example of a race of practical-joke vomit piles, the Horta protects the unhatched next generation by killing miners with acid. In spite of being silicon-based rather than carbon-based, Spock can mind-meld with it, Bones can heal it, and Kirk can seduce it. OK, I'm guessing on that last one.
Powers: Acid baths, rapid tunneling, iffy grammar.
Weaknesses: Type 2 phasers, an implausible life cycle.

Neural Parasite
Episode: "Operation -- Annihilate!"
Description: Looking -- and moving -- like a half-digested Frisbee, these creatures are part of a large organism that takes over humans' nervous systems and forces them to infect other planets, kind of like Borg without the fashion sense. This is also the episode where we learn that Spock has nictitating membranes. That doesn't have much to do with the monster. It's just creepy.
Powers: Flight, mental possession, nigh-invulnerability.
Weaknesses: Light, Spock's impressive yet ill-defined mental abilities.