David Bowser for The New York Times The sculptor John Sherrill Houser standing beside the head of the horse that the explorer Don Juan de Oñate will ride in a monument planned for El Paso. Some historians dispute savageries attributed to Oñate. SANTA FE, N.M. — Four hundred years after he colonized what is now New Mexico for Spain, Don Juan de Oñate (half of him, anyway) lies here with scattered pieces of his horse as if dismembered on a battlefield of titans. But by the end of next year, if all goes according to plan, this helmeted giant will...