It seems some dragons are phs for others. You could consider it some form of cycle, where you need to kill one to spawn others on the same list. Nintal and Yal seem connected in this fashion. At least from my small sample of the past day. I needed to make a note of it somewhere, and I figured here would work best. Someone else can correct me or confirm it.
Edited, Jul 27th 2017 11:41am by Shuraz