EWTN Catholic Q&A
Papal Onomastics
Question from John on 04-03-2005:

Is it true that a Pope may select any name he desires ?

Were there any Popes who used their birth name?

Answer by Matthew Bunson on 04-08-2005:

Indeed, every pope has the opportunity to choose any name he wishes.

Most of the early popes used their original name. Pope John II was the first pope to change his name. His given name was Mercury.