Biology
Three stars...
There was nothing particularly special about that. There were approximately one-hundred billion balls of plasma in this galaxy-it was inevitable that some would clutch together, as if desperate to cast more light over certain areas of the darkness of creation. Besides, the ship that cruised through these stars was far more special...
As the three stars converted hydrogen to helium in spectacular nuclear reactions, the ship did something even more extraordinary. It slowed down...it stopped. Not due to gravity, not due to a bizarre solar wind, but on its own volition. This ship, this creation, was above the stars. It could drain the smallest of them, a small red dwarf in hours had it needed to recharge its energy supplies, energising it from bridge to port, its U shape shining brighter than any natural thing. Yet this didn't come to pass. It had more than enough energy to travel to the next sector in this lonely corner of space and besides, you couldn't seed a star. No, that was reserved for planets-or, in this case, a moon.
Without conscience or regret, the ship's creators prepared for the seeding of the moon orbiting the gas giant that a race of primitives would one day call Polythemus. To them however, it was nameless-one gas giant amongst trillions and completely unremarkable. Just as one of its moons would be, just another hive world in their eternal battle against the Enemy. That their actions would condemn all life on this world to oblivion meant nothing to them-hundreds of their kind died every day, the stars drenched in their blood as well as their foes. What difference could one little moon make?
Like an obscene leach, a hatch opened in the craft, its cargo ready to be deposited. No egg this time-the world below was far too hostile. No, this time they would dispatch their greatest weapon in its fully developed form, bar shifting its biology from male to female in the absence of the latter to guide its actions. A risky move in its own rights, but the creature's ultimate masters simply couldn't afford to drop eggs en masse. Even they had their limits.
However, condemning a world to death was not among them.
On the surface of Pandora, the xenomorph cackled.
Well, technically it was some hissing, schreeing sound, one that would send chills down the spine of any sapient lifeform. Regardless, the essence was there. Oysters might have been located on a world 4.37 light years from here, but it was still fair to say that this moon was the drone's oyster. It was the king of all it surveyed, its black, impervious hide distinguishing it from the hapless flora around it. All that remained was to undergo a few shifts in biology, make itself the queen of all it surveyed and then get around to giving birth so it could put the hapless fauna to good use.
Letting out another hiss, the xenomorph appeared to be...talking? It was hard to say. Maybe it was just angry about something, maybe it was trying to say "Oh yeah! Who's bad? That's right! That's what I'm talking about, bitch!" Kind of like one of the primitives on the same world that oysters were found, flashing their hollow bravado with weapons to match their intellect every time they scared a rhino off.
Or that of the xenomorph. Because hissing and screeching in the presence of a creature 2.5 meters in height and over 5.5 meters in length wasn't the best way to go about turning this world into a hive. And trying to stand up to the creature as it lets out a mighty roar, staring at it with...well, it doesn't have eyes, but anyway...Regardless, bringing its teeth to bear, the palulukan proved that one mouth was better than two and shut the critter up.
"Finally!" exclaimed Eywa, communicating through her most fearsome predator (an actual predator, not one of those psychopathic hunters that showed up from time to time thank you very much). "I thought it would never shut up!"
The palulukan didn't say anything. Nor did it roar. Instead, it seemed to be screaming. And with molecular acid tearing apart the inside of its mouth, manifesting in a dentist's worst nightmare, Eywa had a good idea as to why.
Oh...thought the goddess to herself, realizing that perhaps hiring some na'vi to nip the inflection in the bud might have been a better idea than sending a carnivore. So it has molecular acid for blood...
Whether it be xenomorph or goddess, idiocy was a universal constant.
A/N
While one can see a few aspects of Aliens that could be said to have made their way into Avatar (power loaders to AMPs, both Weyland-Yutani and the RDA are at times called "the Company", etc.), the idea for this came from the movie's script. With the first appearance of the thanator, it mentions how "this thing could eat a T-rex and have the Alien for desert." While the reference didn't go unnoticed, the nerd within me pointed out that eating a xenomorph is probably the last thing anyone wants to do, even if you're at the top of the Pandoran food chain. Thus, the nerd put an idea to the keyboard.