It's pretty much the only one that really rhymes.
However, it appears that people who (mistakenly) think that the song is used that way believe the hens actually represent the three gifts of the magi. Because when I think "gold, frankincense, and myrrh," I think "chickens."
|I was going to put a picture here, but Google Image Search made me uncomfortable.|
So why "French" hens, anyway?
Well, what is a "French" chicken?
That's right, it's a chicken with a beard.
They are the Zach Galifianakis of the poultry world.
They also weren't introduced into England until 1886 (though I'm sure a few showed up now and then before that year), so it's entirely likely that since the song was written in the late 1700s, a "French hen" was a true delicacy and a rare find in England.
But I think there's a better (and dumber) explanation.
See, the English are terrible and naming things that aren't native to England. For example, if you ever wondered why a particular food is called a "walnut," it's because it has the same root as "Wales," a.k.a. "that foreign place" to a lot of English people. The original name for the walnut was wealhnutu in Old English, meaning "foreign nut." So since the Welsh weren't local and the nuts weren't local (they were imported from Africa), the English just used the same word root for both.
So it's entirely possible (and theorized by a few people) that three "French" hens simply mean any three "foreign" hens, since most people back in the late 1700s wouldn't be able to tell you where any chicken they saw came from unless they knew the farm personally.
|"That's gotta be one o' them Swiss chickens, right?"|