This is result of Primal Egg being aged for three days. It always drops one of the three mounts listed below.
The Primal Egg has a chance to drop from random elite dinosaurs on the Isle of Giants. Once obtained, the egg will hatch into Cracked Primal Egg after three days. The cracked egg has a small chance to contain Reins of the Black Primal Raptor, Reins of the Red Primal Raptor, or Reins of the Green Primal Raptor.
Just to note: Zandalari Dinomancers do NOT drop these. They are the only mobs on the island that do not have the egg in their loot table.
Final note: the drop rate of a mount from the Cracked Primal Egg is 100% - it is just random which colour.
(can't edit PTR comments, alas)
Seems that in patch 6.02 you can not open one of the cracked primal eggs. It locks it in place in the bag and can not be moved around.
Yep, I just ran into that too. Now I have a slot in my bag that's completely unusable. I hope Blizzard knows about that.
Additional our Hunter obtained one while killing trash after Horridon in the Throne of Thunder