It's random, you were just lucky to get it in some playthroughs and less lucky in others.
If you want the game to get easier (which it inevitably does anyway, the stronger your hero becomes), why not play on easy mode?
Or if you just can't do without dragons (and I understand the appeal), why not kite (steal navigational maps) your way to Elon, where green dragons are guaranteed, and blacks and reds are also almost always there?