In hundreds of books, games and movies, there are alien species. Sometimes, they are bloodthirsty monsters. Sometimes, they are nice allies who fall victim to our prejudice. Sometimes, they are basically the same as us.
By definition, there is no way to verify which of these aliens are the most realistic, because we have yet to find the real version. But I have my own hypothesis as to how things will go down.
First of all, we have to consider one thing. Following the theory of evolution, we can expect that if there are aliens anywhere, then there will be hundreds of different species, probably all living together. Just like us, there has probably been an ecosystem on their planet at some point, even if it might no longer be the case. We can probably expect there to be a few dominant species, and the rest to be much much less powerful. Just like any alien finding our planet would either see all animal species (including us) as very dumb, or notice that we are above them, depending on where they are on the scale.
We also have to take into account the fact that the first contact will most likely be an information-only contact, which can either be a conversation from afar somehow, or one of the species noticing the other one long before they are themselves noticed. A scenario where we crash land on a planet we thought empty, and end up fighting aliens for survival probably won’t happen. We are literally scouting every single rock we find in the sky as hard as we can right now, and we have no means of ever reaching the vast majority of those. I don’t see how we could ever reach a point where we spend the resources to go directly on a planet without even checking to see humanoid monsters roaming on it.
Of course, there is always the possibility that those aliens will look exactly like something we find everywhere (like rock), and we don’t notice them until we get there. But according to our understanding of physics, if something is advanced enough to be self-aware, we would see some trace of its existence somehow, whether it is heat or movement or something else. If it can hurt us, we can see it.
When we reach the point where one of the civilisations has the means to ever reach the other, then either the other one could too, but did not, or they are very far behind in technology. The Fermi Paradox taught us that our impression of how likely something is can be very very far from the truth, but I still think that across the billions of years that went by since the Big Bang, it would be one hell of a coincidence if two “smart” species happened to develop inter galactic travel around the same time. We haven’t even existed for a million years and we’re already in space after all. I totally expect the first contact to have one obviously and undoubtedly more powerful side, with weapons and tech that the other side never even dreamed of.
So it all comes down to how this Superior side wants things to go. It is possible that they see the others as primitive animals, and force them into zoos or natural reserves (maybe without the other side even noticing, probably without their consent). Maybe they’ll want to keep them as pets, or sell them. Maybe they will try to teach us what they know, assuming they believe we are capable of understanding it, or they have the means to change us enough for that. Or, maybe the superior side completely obliterates the other group instantly because it’s less trouble that way. Maybe they have met countless species like that and know that nothing good comes out of it, like a hunter shooting a bear on sight.
We have to also keep one thing in mind. The Superior side is not necessarily the invader. After a few hundred years of space exploration, humans have finally found a trace of what they thing is intelligent life. Unnatural shockwaves spreading through space. After abandoning a drained earth, all humanity is travelling in a few thousand spaceships as a nomad community. They go towards the source of those signals, hoping to finally meet the aliens we have seen in pop culture for millenia. But they just happen to wander too close to the space equivalent of trench warfare between two galactic empires, and are vaporised instantly without a clue, like a racoon that decided to run through a no man’s land and stepped on a land mine.
Unlike in Starcraft, Star Trek or Star Wars, I highly doubt that actual aliens will ever meet us as equals, whether it is through a friendly handshake or a somewhat balanced fight. Either we will be far above, or they will be, and the best the weak side can hope for is to look like interesting pets enough to avoid annihilation.