This isn't an answer to your question, but it might help getting you what you want. The book of Revelation was not written to Roman citizens, but to Christians. It was written by someone who would have identified himself as a Jewish follower of Christ. So perhaps the question should be addressed to historians who know the understanding of Dragons in Roman Providence of Judea. It would also be interesting to see the understanding of Dragons by the folks who lived in Roman providence of Asia where the letter John wrote was sent.