We've all heard about villains who were only supposed to show up once and ended up sticking around for years—but what about the bad guys who were supposed to live forever and ended up disappearing after one or two outings? Sometimes it's because the menace just doesn't live up to the hype, other times real life, in the form of actor availability or untimely series cancellation, just gets in the way.
Here are ten villains who were supposed to be huge, and weren't.
Mind-Controlling Parasites, "Conspiracy," Star Trek: The Next Generation
Remember when Star Fleet was taken over by a bunch of alien body-snatchers who managed to send off a homing signal before being defeated? A homing signal that would tell their pals back home where to find Earth? Well, you might, but TNG sure didn't.
Experimenting Aliens, "Schisms," Star Trek: The Next Generation
We never knew a lot about these guys, either about the reasoning behind their decision to abduct and experiment on the Enterprise crew or about whether they managed to get off a probe at the end of the episode. The ending is intentionally left open, but after seeing the look of the aliens (producer Brannon Braga described as looking like "fish monks") the producers decided to leave them as one-offs.
The Celestial Toymaker, Doctor Who
The Toymaker, "the avatar of games and illusions," appeared in the (mostly missing) sixth serial of the third season. His M.O. was to force people to play for their freedom in rigged games, and he was supposed to return in a story called "The Nightmare Fair." Unfortunately the show was put on unexpected hiatus, so the script was never filmed, but the Toymaker did go on to make several appearances in things like novels, comic strips, and audio stories.
The Voord, Doctor Who
Created by Terry Nation, the guy behind the Daleks, these wet-suit-wearing villains were supposed to be the next big thing. They were merchandised in hopes that they would reach Dalek-like heights, but despite some initial interest in things like movie rights, they just never really took off, and they never appeared in another televised story (although, like the Celestial Toymaker, they did show up in novels, comic strips, and audio stories).
The Valeyard, Doctor Who
Probably the most famous of Doctor Who's unreturned villains, the Valeyard is "an amalgamation of the darker side of the Doctor's nature" from between his twelfth and final regenerations. Since he was probably supposed to be created by now, his absence is particularly notable, and despite appearing in a bunch of non-television stories, it seems unlikely that he'll return.
The Movellans, Doctor Who
Doctor Who is the runaway winner of the forgotten villain prize (and we're sure that there are plenty more enemies of the Doctor that should have made this list). These very stylish androids were supposedly involved in an intergalactic war with the Daleks for years—which you would think would make them pretty important players, but apparently not, because they just sort of faded away. Maybe it had something to do with wearing their power packs on their belts for anyone to disable?
Logan St. Clair, Sliders
Logan was a double of Quinn's (except for being female), and she was bad news. She murdered her world's Arturo because he discovered that she was planning on using sliding technology to steal resources and hold other worlds hostage, and when the Sliders gang finds out what she's up to, she tries to kill a few other people too. Despite getting an open ending—she ends up sliding randomly too and she may have the ability to find them—she never appeared again.