Dreaming of UFOs or being threatened by aliens portrays how you are exploring aspects of yourself, which you find difficult to 'identify with.' Your sense of being different from the group (and how you feel about it) will be portrayed by how ‘foreign’ the characters appear in your dreams.
Unlike family (genetic or inherited self-dynamics,) friends, people (acquaintances that change you) and even aborigines (the more organic side of you,) aliens would be considered natural creatures, they are just ‘not from around here.’ Being abducted by aliens is actually suggesting how you are being ‘kidnapped' by your fear of conformity and not being authentic. Fitting in with the group often comes at the price of your real nature. A robot would also be offering a message that you are not being authentic or are just going through the motions to fit in. See also Galaxy and Flying Saucer.