I also heard this as a child and it goes:
I had a horse and his name was Napoleon
All I had was the bony parts
I hitched him up to an old milk wagon
Hollered MILK when I wanted him to stop
One fine day while out a-riding
Long came a chap in a rig so neat
He said, You brute I'll run you in a race
All up and down the big main street.
Off we started helter skelter
I had a grin all over my face
Sure as a sinner I was comin' in a winner
When someone hollered MILK and I lost the race!