The Primordial Ouroboros Serpents were creatures of endless hunger with destructive capabilities that were almost unrivaled in all of creation. As 'mortal' beasts, no other known living creature was their equal in this Era.
The purge of the Primordial beasts during the unknown Era in the past had left the crown of the king of beasts to the serpents, and in this land cut off from all reality, the serpents were free to show their brilliance without the eyes of the ancient enemy prying on their secrets.
From the start, Rowan had always kept these beasts on a leash. His Dimensional flesh acted like a cage to contain their power, for as they grew stronger, their growth only added to the weight of his dimension, which ensured that they would ever be suppressed by Rowan.
There was a reason Rowan tried to keep the serpents small, for even he was not sure what it would take to contain them if he allowed them off the leash.