I learned it from a cartoon show: the Beany and Cecil Show. I think Cecil the Seasick Sea Serpent used to sing it.

Then again, from the WAY I learned it, it may have been Foghorn ('I say, I say') Leghorn, because it goes,

R
I say, R A
R A G,
R A G G,
R A G G M O P P
Raggmopp