It's possible that there are or have been real extraterrestrials. I'd highly recommend the YouTube channel ArtAlienTV, which covers very persuasive evidence of a past civilization on Mars.

However, the "aliens" that people have claimed to encounter in abductions and such (greys, reptilians, Nordics, etc), are most likely demons. I read up on them for years and considered all the possibilities (aliens, time-travelling humans, etc), but can't help but keep coming back to the conclusion that they are demonic. Their behavior gives them away, and frankly, the mere physical appearance of the greys screams pure evil to me. It's like I can feel it down to my bones when I see a picture of them.