An elk herd roams the sprawling oak savannas of Northern California with a member who isn’t an elk at all. His name is Diesel, and he is a donkey. The story about how an unassuming pack animal became something approaching a hoofed Tarzan started hundreds of miles away, involved a decades-old federal program and required the heartbreak of two people who had welcomed Diesel into their family.Subscribe to The Post Most newsletter for the most important and interesting stories from The Washington Po