For the doomsday prepper story I am working on, I have a pretty good idea of what kind of creature the colonists will be up against. I have planned out their appearance and capabilities. However, I will not say where exactly they are or come from. They could be aliens, government experiments, living fossils, or inter-dimensional beings. That will remain ambiguous in the story and I will leave it to your imagination. I like monster stories that leave you guessing what the creatures are and where they come from. I will apply that to my story.