We all know how it goes in sci-fi horror monster movies, right? Everything is going just fine and then some mad scientist comes along and splices human and alien DNA, transplants an animal brain into a human body, tries to create a race of fish people, or otherwise does something ill-advised, and then all of a sudden, there's a monster on the loose. The monster might be an alien, it might be a mutant, or it might be some kind of biomechanical horror, but whatever it is, it's causing all sorts of trouble, and everything would have been fine if people had just left well enough alone.
Fortunately, while this kind of meddling may be bad for the characters, it makes for good sci-fi/horror monster movies. In fact, some of the best in the business have followed some variation on this formula, from Alien to The Fly and beyond. Here are a few lesser-known (or just less appreciated) sci-fi shockers that also fit the bill. Vote up your favorites!