I wrote this in five minutes on another forum for a guy who wanted ideas for his own creature.
A creature that is microscopic, basically slug goo, and it has one sharp pincer. You don't realize it happening, it being microscopic, but it pinches you and makes a little hole in your stomach. Enters the hole. Lives in your chest. It feeds off what you eat. It grows. Then, when it's bigger (it will start showing on skinny people when they begin getting pretty fat), and it's ready to go live in the real world, it bursts from your chest. When it comes out, it basically steals your rib cage for protection, and uses it like a shell. It may also use your spine as a tail.
A creature that is microscopic, basically slug goo, and it has one sharp pincer. You don't realize it happening, it being microscopic, but it pinches you and makes a little hole in your stomach. Enters the hole. Lives in your chest. It feeds off what you eat. It grows. Then, when it's bigger (it will start showing on skinny people when they begin getting pretty fat), and it's ready to go live in the real world, it bursts from your chest. When it comes out, it basically steals your rib cage for protection, and uses it like a shell. It may also use your spine as a tail.