The Stone Egg could be seen filled to the brim with Chaos Energy. The energy that Adrian supplied is waking up the creature that is sleeping inside the egg. He already knew that the monkey beast inside will at least have the Monkey King Strain in it which means it can evolve to that. What Adrian does not know is what other strain the egg carries now that it has changed to a mythical tier beast egg.
Mythical tier beast egg is incredibly rare and are only laid by divine beasts since all mythical tier beast will become divine beasts when they reach maturity. The monkey beast that is inside of Adrian's Stone Egg is no exception. The only difference is that the monkey beast will definitely have fewer evolutions compared to when a normal monkey beast is trained from scratch.
