Collage by Andy Warhol titled Untitled (Superman Collage #15), 1960
During the recording of Pisces, Aquarius, Capricorn & Jones Ltd. in 1967, the Monkees set aside time to record an a capella version of a fifteenth-century Spanish villancico carol called "Riu Chiu" (sometimes known as "Riu Riu Chiu"). Why? I have no idea. They recorded the song on the same day they recorded "The Door Into Summer", so maybe they were just in a folky sort of mood. I guess the recording was used in a Christmas episode of the Monkees' TV show, so that kind of makes sense and may answer the titular question in this case.
Now I love the Monkees, but I'll admit to being surprised that they kind of do a good job with "Riu Chiu". It's quite possible that they were mimicking an earlier recording, but their phrasing is unexpectedly classical-sounding and they nail the harmonies. I'm not sure they pull it off with the level of feeling that you'd get from the more devout artists who have covered the song (like Sixpence None the Richer or [I'm not even kidding here] David Archuleta), but their version is definitely more "W-T-F", as the kids say.
"Riu Chiu" by the Monkees