Choosing a 'Pope Name'
The first pope to change his name did so in the Sixth Century because he was named after the Roman god Mercury, and he thought it inappropriate to carry that name as pope. He chose to be called John II. After that some popes changed their names and some did not. The last pope to use his baptismal name was Marcellus II in 1555. Since then, it has been customary for all popes to change their names, although it is not required in any way.
“How Does The Pope Choose His Pope Name?” may be the greatest article title I’ve ever read. The article itself was an interesting read – I hadn’t realized until recently that the Popes change their name upon becoming the Pope.