I'm the Grandpa who used to sing this to my kids and grandkids. Sung to the tune "Reuben, Reuben, I been thinkin".
These are the ONLY TRUE lyrics.

I had a horse and his name was Napoleon
all on account of his boney-parts.
He was sired by an old Hambletonian (a breed of carriage horse)
and darned by everybody round these parts.

He was so thin you could see right thru him
hair as fine as any silk.
So I hitched him up to my old milk wagon
and taught his to stop when I hollered "MILK"

One afternoon when I was out ridin'
along came a feller with his rig so neat.
Said he to me "You old, darn Reuben" (old-fashioned person)
"Let's have a race, right down the street".

So we started, helter-skelter.
I was a-comin at a thunderin' pace.
Sure as a sinner, I was comin' as a winner,
When the feller hollered "MILK", and I lost the race.