I heard it was true (urban myth)

The Queen of England decides she wants a Kentucky thoroughbred in the royal stable, so she calls President Reagan, who decides to meet her in Lexington, Kentucky.

When they get there, they decide to go for a ride. They're just pulling out of the barn when the Queen's horse's tail goes up and "Lbbttt!" - out comes a monstrous fart.

The Queen says, "I'm so embarrassed!"

Reagan says, "You shouldn't be! I thought it was the horse!"

I actually heard someone on FoxNews pass this off as fact, but with Reagan visiting the Queen in England and riding the grounds instead of her coming over here looking for a horse.

After a little looking around (yeah, I thought it was humerous) it turns out that in the 1972 Patrick O'Brian novel Post Captain, there's a drunk guy named Babbington driving a lady in the back of a horse-drawn cart:

-  The horse slowed to a walk - the bean-fed horse, as it proved by a thunderous, long, long fart.

-  "I beg your pardon," said the midshipman in silence.

-  "Oh, that's all right," said Diana coldly. "I thought it was the horse."