For the first time in Pokémon games history, a Pokémon has been added into the game that can be evolved into a legendary form in Pokemon Sun and Moon. The Pokémon in question, Cosmog, isn’t easy to find either and is pretty much considered a secret Pokémon and can only be acquired once per copy of the game.
Cosmog is the first stage of legendary Pokémon Solgaleo or Lunala depending on whether players are playing Pokémon Sun or Pokémon Moon. After leveling, Cosmog evolves into Cosmoem and then into its legendary form.
To acquire Cosmog, players need to complete the storyline of the game and become the Champion of the region. Once they are the champion, they need to return to the Altar of the Sun or Moon, depending on their game version, and enter through the portal present there.
After cross the portal, they will come upon a lake. The lake will be named opposite of the region they are playing on so if someone is playing Pokémon Sun, they will go to Lake of the Moon and vice versa. At the lake, there will be a platform and once players reach that, a cutscene will trigger after which they can add Cosmog to their collection.
Cosmog itself is a pretty weak fighter but leveling it up ends up paying off in the end as the Legendary forms are pretty great. While Cosmog can only be acquired once per game, players can still catch the other Legendary form of Cosmog through the game world so they can complete the entire family line.
Alternatively, they can also use the trade feature to acquire Lunala or Solgaleo from a friend.