Your scans pick up a crashed spaceship on a moon.
If you chose to investigate, you find that the ship was carrying some kind of .. pets? Almost all of them are dead, but you find a surviving pet and take it back to your ship. A moment of carelessness causes the pet to escape on your ship...
Pets could be cats, tribbles, mogwai(gremlins), facehuggers(aliens), critters, etc. Some might fetch you a nice load of scrap, others cause damage to your ship, use a crew member as a host (killing them to spawn a hostile mantis or slug) or eat a crew member, unless you happen to have a teleporter to beam them on that rebel scout trying to FTL away