Located two ½ miles south of Black Horse, San Pedro was a post office designation from November 6, 1911 until September 30, 1912. San Pedro was the site of a mill built in 1911 to serve the Black Horse boom, as Black Horse had no running water. Approximately 15 mill workers and their families lived there. The Black Horse boom was short-lived and the mill closed in 1912. Mill ruins remain to mark the site of this short-lived mining camp.