King's aura referred to the special aura from a King. It didn't necessarily have to be someone with a King Profession but just any creature that had the title of a King, such as Gray Wolf King, Black Forest Horse King, Wild Boar King, etc.
Any creature with 'King' in its name would have King's aura, and the more powerful the creature was, the more powerful its King's aura would be. Conversely, the weaker the creature was, the weaker its King's aura would be.
For example, for creatures like a Wild Dog King or a Wild Chicken King, they would possess King's aura but very little of it.
At the same time, people also had King's aura. They didn't necessarily have to have a King Profession; rather, as long as they had a Royal Bloodline or were the Legatee of a King, they could be included.
In other words, direct descendants or Legatees of Kings would also have King's aura.