(Casey Riffe | Billings Gazette | The Associated Press) In this June 9, 2010, file photo, a mare nurses her foal in the Pryor Mountain Wild Horse Range near Pryor, Mont. Animal rights advocates are suing the federal government in a bid to make a Montana mustang population the first group of wild horses to be protected under the Endangered Species Act. Attorneys for Friends of Animals said Wednesday, April 4, 2018, that the U.S. Fish and Wildlife Service had failed to act on a petition filed last June seeking protections for the population of about 155 horses along the Montana-Wyoming border.