One of the more fascinating things about language is that it changes gradually over time as to be almost unrecognizable. So, an old song like “The Fox” (Roud Folk Song #131) is particularly interesting because it is a song in Modern English that predates Modern English. The version of the song I know best goes like this:

The Fox

The fox went out on a chilly night
Prayed for the Moon to give him light
For he’d many a mile to go that night
Before he’d reach the town-o town-o town-o
Many a mile to go that night
Before he reached the town-o

He ran till he came to the farmer’s pen
Where the geese were kept therein
“A couple of you are going to grease my chin
Before I leave this town-o town-o town-o
A couple of you will grease my chin
Before I leave this town-o.”

He grabbed the gray goose by the neck
Slung a little chick over his back
They all screamed, “Quack quack quack,”
With their legs all dangling down-o down-o down-o.
They all screamed, “Quack quack quack,”
With their legs all dangling down-o

Then Old Lady Flipper-Flopper jumped out of bed
Out of the window she stuck her head
Crying, “John, John, the gray goose is gone
And the fox is on the town-o town-o town-o
John, John the gray goose is gone
And the fox is on the town-o.”

John he ran to the top of a hill
Blew his horn both loud and shirll
The fox said, “I’d better flee with my kill
For he’ll soon be on my trail-o trail-o trail-o.”
The fox he said, “Better flee with my kill
For he’ll soon be on my trail-o.”

The fox ran to his cozy den There were the little ones eight, nine, ten
They said, “Daddy, you’d better go back again
For it must be a mighty fine town-o town-o town-o
Daddy better go back again
For it sure must be a mighty fine town-o.”

Now, the fox and his wife without any strife
Cut up the goose with a fork and knife
They never had such a supper in their life
And the little ones chewed on the bones-o bones-o bones-o
They’d never had such a supper in their life
And the little ones chewed on the bones.

The origins of this piece are lost in time, but there exists a copy of it in Middle English located in the British Museum’s library. Note the similarities and differences from the modern version (I've translated some words that might be unfamiliar in the pipelinks):

The Fox ycomen

“Pax vobis,” qoud the fox.
“For I am comen to towne.”

It fell ageins the next night
The fox yede to with all his mighte
Withouten cole or candlelight
Whan he cam unto the towne

Whan he cam all in the yarde
Sore the ges were ill aferde
“I shall make some of youre berde*
Or that I go from the towne!”

Whan he cam all in the crofte
There he stalked wunderfull softe
“For here have I been frayed full ofte
Whan that I have come to towne.”

He hente a giise all by the eye,
Faste the goos began to creye!
Oute yede men as they might hete
And seide, “Fals fox, ley it downe!”

“Nay,” he saide, “so mot I thee--
She shall go unto the wode with me,
She and I under a tree,
Emange the beryes browne.”

“I have a wyf, and she lieth seke
Many smale whelpes she have to eke.
Many bones they muste pike
Will they ley a downe.”

The first thing I’ll note is that the songs have the same cadence. When you say both middle and modern versions out loud without adding the tune they both have the same alternating stress pattern. They also seem to have the same rhyme structure:

A night
A light
A might
B town-o

What is different is illuminating. Middle English has no silent letters and this poem is no different. With the exception of “mighte” in the fourth line all the end E’s are voiced. Therefore “towne” sounds like “town-ah”. When Middle English gave way to Modern English, the extra E was dropped, but if my hunch is right it was not dropped from this song.

A well-known folk song with a “town-ah” at the end likely would stay close to its original form, but eventually it seems that the “ah” was reinterpreted as an intensifier for a refrain and modified into “O” thus “town-o”. When we look at where the rhyme scheme differs in the modern version, it is almost entirely in the repeated refrain:

A The fox went out on a chilly night
A Prayed for the Moon to give him light
A For he’d many a mile to go that night
B Before he’d reach the town-o town-o town-o
A Many a mile to go that night
B Before he reached the town-o

Half of it has been added almost purely based on a mistaken assumption that town and O are separate words.

I find this a very interesting example of how a poem with a little unconscious adaptation became one of the oldest songs in the language.

*It probably means "Beard" which means the same thing as the Modern English poem does at this point "Grease my beard/chin".

The text of "The Fox ycomen" is from http://www.folklorist.org/song/The_Fox. The text of "The Fox" is from memory.

Other additional sources:

Chaucer, Geoffrey, and Larry Dean Benson. The Riverside Chaucer. Boston, MA: Houghton Mifflin, 1987. Print. (Especially the Introduction for Middle English pronunciation.)

Sisam, Kenneth, and John R. R. Tolkien. A Middle English Reader. Mineola, N.Y: Dover Publications, 2005. Print.