Until the Goryeo Dynasty, only the royal family and the most powerful nobles of the time had surnames.
In the history of Goryeo itself, it is simply written that Wang Geon’s father was Yong Geon and his grandfather was Jak Je Geon. It is not written as Mr. Wang anywhere.
Of course, at that time, there were people whose names included ‘King’, so it is possible that Wang Geon’s last name was Wang.
Considering the world before the Goryeo Dynasty, it is highly likely that the king was not a surname.
In other words, Wang Geon = name
So, the progenitor of the Kaesong Wang clan is Wang Geon.
For reference, surnames began to be used in earnest on the Korean Peninsula when Wang Geon began bestowing surnames.
It is not yet clear whether the real surname is Wang or whether the name was Wang Geon without a surname.