Rotworm

A type of maggot which eats living flesh, consuming it from the inside and eventually becoming the muscle and tendons of the body, able to control it. The creatures are only able to reproduce in the conditions inside a living body, and so they keep the organs functioning as best they can, still eating through the original creature's mouth and utilizing its organs to maintain some semblance of life. It has been posited that the entity itself remains alive and conscious throughout this process.