* If you're talking about a specific dragon: Let's say you're referring to Smaug from The Hobbit. In this case, Smaug would be a concrete noun because it refers to a specific, individual dragon.
* If you're talking about dragons in general: Then "dragon" would be an abstract noun. It's a general idea or concept, not a specific, tangible being.
So, the answer to your question depends on the context in which you're using the word "dragon".