After defeating the Elite Four, you will be shown a scene of Tapu Koko calling to your player. After seeing this, you can go to the Ruins of Conflict and interact with the statue at the Altar in order to summon and battle Tapu Koko. Tapu Koko will be at Level 60, knowing the moves Nature's Madness, Discharge, Agility, and Electro Ball. You will receive Tapunium Z after capturing it.