Speculation:Kieron
From the Lyorn Records of the Dragaeran Empire
It is possible that Kieron may have been the first Dragon Emperor.
Kieron presumably went to war under the rule of Zerika the First. This war could have taken hundreds of years, and it wasn't until AFTER the war that the Jhereg were made a permanent part of the Dragaeran Empire.
Aliera says Kieron was killed "Later" by a group of "Lyorn Warriors and Shaman". Given the way Dragaerans think about time, "Later" could be 600-700 years after the formation of the house of Jhereg...
And the fact that the Lyorn played a part in his demise makes me think that maybe this was the event that caused the first turn in the cycle from the Dragon to the Lyorn?
We have to allow at least 289 years for Zerika I to rule, then at least 289 more for Kieron under this scenario, and probably more since two minimum-duration reigns back-to-back is likely to be pretty uncommon.