Not an alien being, but a clever bracing system which adds the necessary strength to the back but allows the back to properly vibrate. While the top wood produces the MOST sound, the back adds a lot to the entire mix.
A most famous guitar builder of the 1800's, Antonio Torres, reportedly built a guitar with the back and sides made from paper mache' to prove a point about the top wood producing the sound. It supposedly sounded good. However the guitars of that period were so QUIET they were hard to hear except being very close to the player.
Today, we expect more volume and indeed demand it, hence the back needs to contribute as well, hence the cool bracing.