8. Star Trek - "Mirror, Mirror"
Even if you've never seen a single episode of Star Trek: The Original Series, even if the only person you identify as James Kirk is Chris Pine, you know that Spock with a goatee is evil. That's just the way things work. Thanks almost entirely to Star Trek's Mirror Universe, we now know that facial hair is, without exception, evil. But what is this Mirror Universe? Well, pretty much exactly what it sounds like. In one episode of Star Trek, they encountered a world where everything was exactly the same as their own universe, except for the moral standing of the main characters. As opposed to the generally peaceful nature of the Federation as it exists normally, the mirror universe is constantly ravaged by war. Correspondingly, the characters in the mirror universe are significantly more suspicious and aggressive than their counterparts. The United Federation of Planets has now become the brutal Terran Empire, run by humans and Vulcans together. Instead of exploration, which was always the main function of the Federation, the Terran Empire focuses entirely on conquest. This Mirrorverse is notable partially because it is a strong early example of this trope (and its namesake, for that matter), but let's be honest, it's mostly because of Mirror! Spock's goatee.