If the Targ dragons are dead, there is a chance one will spawn in Asshai. An event will pop up offering the chance to release the dragon, try and slay it (bad relations with dragonriding targs results if you succeed) and to tame it (depending on skills, you have a high chance if injury, failure, and death by dragonfire). If you are in the Seven Kingdoms and are appointed to the small council, you can try to 'face the dragon' if any reside there and are riderless.