Cosmog is a central part of Pokemon Sun and Moon. It evolves into Solgaleo or Lunala, depending on which version you bought. You will naturally encounter the Legendary Pokemon in your version, but completionists would want to catch a Cosmog that directly evolves into Cosmeon to register them to their Pokedex. However, catching it is not as easy as just roaming around Alola.
Catching a Cosmog in Pokemon Sun and Moon would require you to step into another dimension. As long as you follow this guide, though, you will 100% receive your secret Cosmog. But note that you are entitled to receive only one per copy of the game.
Just Follow The Storyline
Don't think about getting a Cosmog at first, you will not be able to no matter where you look. Just go on your way and complete the main storyline before you even attempt to find it. That goes to say that you should become a champion of Alola.
After you finish the story, head over to the altar where you caught your Box Legendary. It's Altar of Sunne in Pokemon Sun and Altar of Moone in Pokemon Moon. Interact with it.
Fly To The Other Dimension
A portal should appear that would take you to a different world with reversed time of the day. You cannot enter without Solgaleo or Lunala, which is why you would need to finish the main storyline first. If you have Solgaleo, the wormhole opens at night and vice versa for Lunala.
Then, fly to the lake that is opposite your altar. If you are playing Pokemon Sun, then head to Lake of the Moone and Lake of the Sunne if you are playing Pokemon Moon. You can also gain access to these locations if you go to Ula'ula Meadow.
Get Your Secret Cosmog
Once you reached the altar with the Box Legendary still in your party, you can now get a Cosmog. You don't have to do anything; just walk up and a cutscene should appear that will allow you to add it to your party or box.
If you want to use it competitively, then save before going to the altar. You can perform a soft reset of Pokemon Sun and Moon if you want different IVs and natures. You can also save Cosmog for later as you can go through the portal again and again.
What To Do With Your Cosmog
You can level it up to 43 so it will evolve into Cosmeon, which then evolves into either Lunala or Solgaleo at level 53. Many have commented that it is a rather weak Pokemon in Pokemon Sun and Moon, but you have to grind it nonetheless if you want to complete the Pokedex.
A few things to note, though. Coming from another dimension doesn't mean that the Cosmog will evolve into the other Legendary. It will still evolve into what's on the cover of your game. If you want the other one, you can always find someone to trade with.
Meanwhile, check out our other useful guides. How to Hatch Eggs Faster; How to Get a Female Salandit; Team Skull's Passwords; How to Catch All Four Guardian Deities; and How to Transfer Pokemon from past titles.
