Jonesy, the ships cat.
The crew set down on a hostile planet to investigate what is thought to be an SOS. They end up under attack from an alien creature, hiding in the bowels of the ship. Something that Jonesy watches with detachment. Survives thanks to Signorney Weaver's character Ripley who tracks him down and takes him with her when she blows the ship up. So good he returned with her in the sequel.