Turns in to Cracked Primal Egg after 3 days, which always drops of one these mounts:
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 Primal Egg has 100% chance to contain either Reins of the Black Primal Raptor, Reins of the Red Primal Raptor, or Reins of the Green Primal Raptor.
Additionally, Zandalari Dinomancers do NOT drop these. They are the only mobs on the island that do not have the egg in their loot table.