SweetShark said:
Yautja. They're called the Yautja.
If we see for example an Xenomorphs, we understand that it have mouth, two arms, legs etc. Plus if we watch the movies of Alien series, we see the Xenomorphs are like animals trying to accomplish the most basic instics: eat and expand their species. Plus we can "explain" their actions. We can explain how the Xenomorphs breed their race for example
I would argue that you actually chose a terrible example. In reality, we don't really know much of anything at all about the Xenomorph. We can make guesses, sure. But pretty much nothing is concrete.
It's been hinted in the movies that the Xenomorphs
don't eat their prey since they wouldn't be able to use their prey as hosts if they did, so what do they consume? Especially curious since they seem to be constantly excreting an unknown substance from their mouths as well as the unknown substance that they make their hives out of. Notably, in each of the movies it's also shown that they don't seem to eat the corpses of their prey after the Chestbursters have come out; they just leave the bodies strung up on the walls.
Additionally, it's unknown whether or not they actually need sleep. Some people infer that they're nocturnal because of Newt's "they mostly come at night, mostly" line, but it's still an unknown. It's been posed quite frequently that they only hunt at night not because they sleep during the day, but because it's easier for them to strike their victims and disappear at night.
It's unknown whether or not they actually need to breath like we do, or what they breath. While the Queen at the end of Aliens, for example, is clearly seen and heard to be breathing, the official novelization of the movie mentions that the Queen isn't actually killed when it's ejected from the Sulaco's airlock. In fact, there are a few non-canon comic and novel storylines in which that particular Queen from Aliens goes into hibernation as it floats around in space until being picked up by passing ships that conveniently happened to find it.
It's unknown whether or not the Xenomorph was found or created by the Engineers. It's implied by the themes of Prometheus that they were created, but it's still rather murky. There's nothing concrete on where exactly they're from. Additionally, with how the Xenomorphs take on traits of their hosts, we're still not exactly quite sure what an "original" Xenomorph is actually like other than the germs in the black goop found in the canisters from Prometheus. Which again... created, or found? Created from what, or found where?
We have no idea how they communicate. It's implied in both Aliens and Alien: Resurrection that they communicate telepathically. It's also implied that they're a hive mind. Yet it's also implied that those last two implications are only true when in close proximity to a Queen, and that lone Xenomorphs are capable of independent thought.
We can't explain their actions, because we're not entirely sure what their motivations are. Sometimes their motivations seem to be nothing more than survival. Sometimes their motivations seem to be expansion. Sometimes their motivations are implied to be little more than malice. Other times it's implied that they don't actually have motivations whatsoever and they're just "programmed" to function the way they do as a bioweapon.
We have no idea how intelligent they are, or how much they're capable of comprehending. At times it's implied that they're just as intelligent as an average animal or insect, and other times they're setting up traps, cutting power/lights, and showing signs of actual tactical planning.
I could go on, but I won't. I've already written a huge wall of text that no one's gonna read anyway.
