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.
Update: 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)