Alien vs. It! The Terror From Beyond Space
Most people don't know this,but Alien is widely considered to be a remake of It! The Terror From Beyond Space.
I've assumed for years that the LV-426 lifeforms were bred in order to fight the creature(s) in that film,for the following reasons:
(1)Acid blood is primairily a close-range weapon.However,against a space vampire such as It!,it is a great defense.Also,consider the strength of the acid-in the first movie it almost ate through the Nostromo's hull!!! Remember, It! could stand up to bazooka fire ,so it would take a VERY strong acid to hurt It.
(2) The creatures from the Alien movies didn't seem to require oxygen.Considering It could survive on the surface of Mars for long periods,this would be a necessary ability for them to have.
(3)The LV-426 Aliens were clearly intended to reproduce VERY RAPIDLY.As even they would take heavy casualties fighting one It,rapid reproduction would be vital.