Aliens

Aliens are beings not from Earth or the main planet of a series - they can vary from terrifying monsters ruled by animal instinct to genius-level super-beings or proud warriors, it was popular in the early days of sci-fi to have aliens resemble humans and they were almost always evil (save for in series such as Star Trek): however as sci-fi progressed the idea of good aliens became more popular (especially after ET) - also after the filme "Alien" came out it became more popular to have parasitic monster-type aliens that were not human in looks or behavior.