the funniest joke

The following is transcribed from the Alan Plater "Oliver's Travels" audiobook (read by Alan Bates), which differs in many minor details from the TV version. In the televised mini-series, this joke is referred to but never told.
The Horse That Liked to Sit on Eggs

"ONCE UPON A TIME there was a farmer who had a horse. His neighbor admired the horse and offered him ten pounds for it. The farmer accepted the offer.
However, before they shook hands on the deal he said to the neighbor, "I should warn you: this horse has one serious weakness: he likes to sit on eggs. So whatever you do, keep him away from eggs."
The neighbor said yes, he would keep the horse away from eggs. So the deal was struck, the neighbor paid the money, and took the horse away.
A week later, the farmer heard loud cries coming from the river. He went down to the river, and there was his neighbor, on the horse, in the middle of the river.
"He won't move," said the neighbor. "We've been here all morning."
"I'm very sorry," said the farmer. "I should have warned you. I told you that he likes to sit on eggs, but I forgot to tell you that he also likes to sit on fish."