Since an invisible dragon would be part of nature, it has a cause, so even if there was an invisible dragon living in your garage, that doesn't do damage to the proof of the fact that since nothing in nature is without a cause, therefore the uncaused Creator must exist.
What if they claim that the Dragon has been for eternity and is the creator of everything? Would that give the claim anymore truth because the dragon is without cause and it is a creator? If you argue that then you could claim...well anything, and the claim requires no proof.