Could a horse beat a man in a marathon? A pub owner in Wales turned a barroom debate into an annual event: The Man Versus Horse Marathon. WSJ's John Miller reports from Llanwrtyd Wells.

But marathons and ultramarathons are a whole other ballgame. Each year, a small town in Wales holds the Man Versus Horse Marathon. It's a 22-mile race between riders on horseback and runners.

That would be two things. It’s the name of a horse that has been chewing up marathon races over the last two years. And it ...