Here’s what might be the original mash-up: Bing Crosby and David Bowie singing the Little Drummer Boy.

This first aired in 1977. The Washington Post did a story on it in 2006. They say:

One of the most successful duets in Christmas music history — and surely the weirdest — might never have happened if it weren’t for some last-minute musical surgery. David Bowie thought “The Little Drummer Boy” was all wrong for him. So when the producers of Bing Crosby’s Christmas TV special asked Bowie to sing it in 1977, he refused.

Just hours before he was supposed to go before the cameras, though, a team of composers and writers frantically retooled the song. They added another melody and new lyrics as a counterpoint to all those pah-rumpa-pum-pums and called it “Peace on Earth.” Bowie liked it. More important, Bowie sang it.