At nighttime in a remote region of western Mongolia, Donald Trump Jr. used a rifle with a laser sight to shoot and kill an endangered argali, the largest living sheep. Local hunting guides fanned the lights of their cellphones across the ground to search for where the ram-like creature fell. Trump Jr. asked them not to dismember the animal on the spot, but instead to carry it away on an aluminum sheet to keep its fur and horns intact.

ProPublica described the August excursion in…