BUENOS AIRES - They look like ordinary foals, docile with honey brown coats and white facial patches, content to spend their days munching alfalfa in a cordoned-off pasture in rural Buenos Aires province. But these five 10-month-olds are the world’s first genetically edited horses: cloned copies of a prize-winning horse named Polo Pureza, or Polo Purity, with a single DNA sequence inserted using CRISPR technology with the aim of producing explosive speed. Kheiron Biotech, the Argentine company that created the horses, says gene-editing has the potential to revolutionize horse breeding. While cloning creates a genetically identical copy, CRISPR functions as...