Exist Unused controls for alien and marine (probably for predator too). It is not mapped to any keys and can't be mapped in code it has no link to the keyboard keys. But if modifying the game a little bit keys can be bound to perform these actions.
Marine:
1. Unused action is "Crouching", mostly not affect on gameplay, like normal crouching in FPS game.
Alien:
1. Unused action with a tail. This ability makes you move slower.
It doing something with the tail (charging/blocking?). Animation named "Alien_hud_tail_ready_idle".
2. Alien can spit acid. Shots 5 balls of acid. Those are different from jungle alien acid balls.
They have landing sound, spitting sound is different, and their movement physics differs too.
You can fire up to 50 shots.
Videos:
Spoiler