I remember being fascinated at a John Roberts and Tony Barrand concert during the 80s at The Ark in Ann Arbor, Michigan, when/where they explained that The 12 Days of Christmas had once been a "macaronic" song (i.e. a song sung in two languages), and that the line had originally been (as Reinhard quotes from Wikipedia, above) "a partidge, une perdrix". It particularly makes sense to me when you realise that the "e" on the end of "une" is pronounced as its own syllable, making "une" a two-syllable word (something like "ooh-na"). I can see how "ooh-na pare-dreex" could so easily become the mondegreen "in a pear tree"...