There's an extremely difficult quest that one can do in order to obtain a dragon, rated either deadly or epic level.  Right now, however, there seems to be an issue with AI sovereigns automatically completing any quest they go on, no matter how difficult.  This is probably how Magnar got a dragon so early in the game.

Note that I'm assuming this is still early game, before turn 100.  Otherwise, HF's reply works.

This thread was dedicated to the issue.  There's very little you can do to stop an Ashwake dragon before turn 50 or so, and even then, it's only if you have designed your side for it.