Where did the concept of the dragon come from?

I know unicorns were born from sailor's stories about the Indian rhinocerous. Can someone tell me where the dragon came from? Is it possible for one type of dinosaur like creature to have survived, just as some zoologists have theorized some birds might have. Perhaps it did not look quite like a dragon. I know it did not breath fire and that may be its symbolism of terror. Could it be the crocodile? Thanks.