This is not a definitive answer but my experience.
I tammed my Draptor after 7 tries and completed the bestiary after tamming it. I've found most of the draptor at Razachai Palace.
The first place where you can find them is here but if you don't find any there try the second place here. When the spawn occurs both place will have Draptors but the first is the path to the quest, so it is possible that someone killed it without checking the second place.
About the frequency. Checking it every night once a day I've found them once every 1~2 weeks. The spawn is not so rare. Also I've always found 4 of them when both places spawned.