From the experts:
When the King was alive, and for years afterwards, few people (except Declan McManus who became Elvis Costello) dared use his singular name, but now it's very much up for grabs.
The name Elvis is connected to the Irish Ailbhe, and the Irish saint of that name is also known as Elvis of Munster.
Elvis was the middle name of the singer's dad, Vernon. Anthony Perkins, a big Elvis fan, gave the name to his now grown son in 1976. Other aficionados, such as Cindy Crawford, have opted for the surname Presley.