I believe the distinction between immigrants and settlers is settlers come to create something when nothing is there (debatable since people have lived in the Americas for thousands of years, but from the perspective of a European-style civilization, non exists). Immigrants are people coming to an established society.
I think where I draw the distinction for settler versus indigenous is that if you truly were the first to establish something in a region and build a culture/civilization in that region, then you're indigenous to that region. I think how long you've inhabited that region also plays a part, thousands of years certainly is long. I guess the argument then becomes whether the civilizations you find were truly the first and not simply conquerors that stamped out the "true" indigenous people. My attitude, until you have evidence support that, I say leave the unicorn army out of it.