I have no idea what it could be, but let the pod dry really really well. The seeds need to mature inside, so wait for the pod to turn brown. This could take a while. You may want to mark it somehow, and you may want to protect it in some way so that you'll get the seeds when the time comes. Some plants spew out their seeds when they're ready; they may self-sow (and hey, that would be kinda cool.