In 1970 Arthur Rankin Jr. and Jules Bass co-produced "Santa Claus Is Coming To Town." It's a fun family video that explains it all... well, one version of it "all".
In the beginning it shows a baby abandoned near the Kringle's cottage in the woods. The only identification on him was a note that said "Claus". The Kringle family took the child in and named him Kris. As Kris grew older he began delivering toys to the children of Sombertown - to the BurgerMeister's dismay. Eventually, it became law that the children of Sombertown could never have toys again, but Kris Kringle kept delivering them anyway. He soon became a wanted man and had to change his name to the only one that seemed right... "Claus".
Just think of it. Our jolly old Saint Nick, who works hard all year long to bring the children of our world enjoyment on the holiest night of the year, is not only a criminal on the run, but he's also an adoptee!