Spent a fair amount of time in Mons Letalis farming rockhopper hides and have never seen the Ancient Stonegrabber or Ancient Horror. The Ancient Rockhopper is fairly common although after about 15 kills the only thing it has ever dropped for me has beend a Damaged Rockhopper Hide =/ Appears to always be 2 or 3 named hoppers/stonegrabbers/horrors up and when one is killed it's replaced by another minor named. It's a shame the items received from the Twilight Armor quests don't reflect the time/difficulty of getting the drops required.
I'll just have to keep a closer eye on tracking I guess if I want to add this to the kill list.