A druid player who was farming these things near me told me, after saving me from a big nasty elite Dragonkin I was stupid enough to try to whack myself (I was told the dragonkins had the Black Drake's heart too), that he had been killing whelps here for the last four days, trying to get any Whelpling as a pet for a Christmas present. The Dark Whelplings have a 1 in 1200 chance of dropping from a killed whelp.
I felt so bad about it, I just said "Good hunting" and left quietly with my Whelpling after I picked up my quest item. :P
My beginner's luck, I guess. Good, I need the gold anyway, tempting as it is to keep it as my own pet.