I believe in mythologic creatures, scientists and people say they don't exist, no proof that they did (except myths) but still no proof that they DON'T exist.
A dragon might have been a strange creature and it was given the name cause it frightened people by it looks or so.. and so on rumors about fire breathing dragons grow. A few dinosaur explorers (dunno the english word) found a few years ago bones from a strange dinosaur looking like a Dragon. If people from medieval times found those bones too, they might have though a demonic creature just died there recently and brought the rumor about new monster
WHO KNOW?