Extra-terrestrial beings in spiritual terms are those that do not have the same qualities as human beings. They are not ‘of the ordinary’. Such images suggest that we have access to other realms of being in the dream state.
Psychological / emotional perspective: Emotionally, anything that is beyond our normal experience may be depicted as extra-terrestrial. We need to give such emotions some kind of form in order to be able to deal with them. It is possible that in feeling different to others we sense ourselves to be alien and not of this world.
Material aspects: From a practical perspective, there is no reason to suppose that extraterrestrial beings do not exist, or that there is no such things as ufos (unidentified flying objects). However, opinions vary as to whether the objects and beings we encounter in dreams are figments of the dreaming mind or aspects of our own personality, or are indeed extra-terrestrial.