At the Hellabrunn Zoo, in Munich, Germany, a small herd of short-maned, pony-sized horses grazed contentedly last week. Fine-boned and broad-skulled, with mouse-grey coats and zebra-striped legs, they look for all the world like tarpans, the fierce wild horses that Roman legionnaires found in Spain. But tarpans are extinct: the last herds vanished in the 19th century, after ranging eastward to the steppes of the Ukraine.
The Hellabrunn horses are a modern throwback to that ancient breed—the end products of careful experiment. Unlike the average horseman, who works at improving the breed, Geneticist...