There are a few variations in different versions. Basically, it goes like this:

I had a horse his name was Napoleon
All on account of his bony parts
He was sired by old Hambletonian
Damned by all who drove his carts.

He was so thin you could see right through him
Hair on him was fine as silk
I drove him to my milk wagon
Taught him to stop when I hollered "MILK"

I was out drivin' one fine mornin'
Along came a rooker, rig so neat
Says he to me, come on old duffer
We'll have a race right down the street

Oh, I started out all helter skelter
I had a smile all over my face
Sure as a sinner I was comin' out the winner
Then the cuss hollered "MILK" and I lost the race