If you do find a real Dragon, please take some pictures [close up and not blurry would be best] I will go so far as to say maybe there were once dragons like there were once dinosaurs and dodos.
Research Dragon mythology, not all dragons look like the European Dragon, some have no legs, no wings, feathers, vary in size and shape, breathe fire, spit venom and on and on and on. Not every culture called them 'Dragons' either, through translation the easiest explanation would be 'Dragon'.
Like legends of werewolves and tales of witches flying on brooms, there is a logical explanation. With werewolves there's a condition that causes extra hair to grow, to medieval man they could assume it's a curse instead of a medical cause. Witches would jump over broomsticks depending on the ritual, curious onlookers would claim they were flying for two reasons, one in the dark their eyes could of been deceived, two, it sounds scarier they were flying. [Same with why real witches aren't in movies, two hours of people meditating would get boring]