Location: Altar of the Sunne (Sun), Altar of the Moone (Moon)

The main Legendary Pokémon in the games are actually ones that have been with your travelling companion, Lillie, all throughout the game. Starting out as the Cosmog you rescue on the Mahalo Trail, it grows attached to you. Once kidnapped by the Aether Foundation, it evolves into Cosmoem and sends Lusamine to Ultra Space. Lillie asks you to go find the Legendary Pokémon in order to create a portal to get her back. However, when doing so, Cosmoem goes into the beam that summons it and evolves into Solgaleo or Lunala depending on the version, and helps you go to Ultra Space to rescue Lusamine. After doing so, Solgaleo/Lunala wants to join Lillie, but with Lillie not being a trainer or going on adventures, she suggests it goes with you and requests that you capture it. It has to be captured.

Solgaleo Level 55

OT: Yours ID: Yours Ability: Full Metal Body Hold Item: None

??? Nature. Date of Receiving Altar of the Sunne. Caught at Lv. 55

Sunsteel Strike Cosmic Power Crunch Zen Headbutt

Lunala Level 55

OT: Yours ID: Yours Ability: Shadow Shield Hold Item: None

??? Nature. Date of Receiving Altar of the Moone. Caught at Lv. 55

Moongeist Beam Cosmic Power Night Daze Shadow Ball

Location: Lake of the Sunne (Sun), Lake of the Moone (Moon)

With the Cosmog you befriended in the game having evolved into the cover Legendary, it is often questioned how to get another Cosmog. This Legendary Pokémon, while technically an Ultra Beast, is a rather complex one to get. After you have defeated the Elite Four, when it turns to Night in Sun or Day in Moon, you can return to the Altar of the Sunne/Moone with Solgaleo/Lunala in your team respectively and you'll be able to go to the other world, where it is back to Day/Night depending on your game. Then, go to the Lake of the Sunne or Lake of the Moone from Ula'ula Meadow and reach the altar, with the cover legends still in your party. Cosmog will then appear. You don't need to battle it, it will join you automatically. It can evolve into Cosmoem and then into another Solgaleo in Sun or another Lunala in Moon.

Cosmog Level 5

OT: Yours ID: Yours Ability: Unaware Hold Item: None

??? Nature. Date of Receiving Lake of the Sunne/Lake of the Moone. Caught at Lv. 5

Splash

Location: Route 16

Getting Zygarde is one of the most unique, and cumbersome, ways to get a Legendary Pokémon. To get it, you need to collect Zygarde Cells Click for details and take them to the special caravan in Route 16. These cells will depend on what you can do. Until you have collected all the cells, you'll be able to create Zygarde at the Zygarde Restoration Center. This allows for you to put together a Zygarde. If you have 10 cells, it will create a 10% Forme Zygarde, and if you have 50 it will create a 50% Zygarde. These have Aura Break. These Pokémon can then be used to assemble or separate the Zygarde. If you separate Zygarde, the Pokémon will be lost entirely, but can be rebuilt just with different IVs and no EVs. The level increases with the progression in your game.

After you have collected all 100 cells, you'll be able to combine them more solidly into a Zygarde with the Power Construct ability. This will then allow it to change between 10% and 50% Forme, and turn into Complete Forme in battle.

Zygarde Level 30

OT: Yours ID: Yours Ability: Aura Break Hold Item: None

??? Nature. Date of Receiving Route 16. Caught at Lv. 30

Safeguard Dig Bind Land's Wrath

Zygarde Level 50

OT: Yours ID: Yours Ability: Aura Break Hold Item: None

??? Nature. Date of Receiving Route 16. Caught at Lv. 50

Bind Land's Wrath Sandstorm Haze

Zygarde Level 50

OT: Yours ID: Yours Ability: Aura Break Hold Item: None

??? Nature. Date of Receiving Route 16. Caught at Lv. 50

Bind Land's Wrath Sandstorm Haze

Zygarde Level 50

OT: Yours ID: Yours Ability: Power Construct Hold Item: None

??? Nature. Date of Receiving Route 16. Caught at Lv. 50

Bind Land's Wrath Sandstorm Haze

Location: Ruins of Conflict

After defeating the league and becoming the first Alola Champion, in the festival being held to celebrate, Lillie will suggest going to the Ruins of Conflict. When there, she will urge you to make a wish at the pedestal. When you do, Tapu Koko will challenge you. If defeated, it can re-appear after defeating the Elite Four

Tapu Koko Level 60

OT: Yours ID: Yours Ability: Electric Surge Hold Item: None

??? Nature. Date of Receiving Ruins of Conflict. Caught at Lv. 60

Nature's Madness Discharge Agility Electro Ball

Location: Ruins of Life

After defeating the league and becoming the first Alola Champion, you'll be able to go to the Ruins of Life once more. When there, if you touch the statue at the pedestal, then the Akala Island Guardian will challenge you

Tapu Lele Level 60

OT: Yours ID: Yours Ability: Psychic Surge Hold Item: None

??? Nature. Date of Receiving Ruins of Life. Caught at Lv. 60

Nature's Madness Extrasensory Flatter Moonblast

Location: Ruins of Abundance

After defeating the league and becoming the first Alola Champion, you'll be able to go to the Ruins of Abundance, beyond Haina Desert, once more. When there, if you touch the statue at the pedestal, then the Ula'ula Island Guardian will challenge you

Tapu Bulu Level 60

OT: Yours ID: Yours Ability: Grassy Surge Hold Item: None

??? Nature. Date of Receiving Ruins of Abundance. Caught at Lv. 60

Nature's Madness Zen Headbutt Megahorn Skull Bash

Location: Ruins of Hope

After defeating the league and becoming the first Alola Champion, you'll be able to go to the Ruins of Hope, once more. When there, if you touch the statue at the pedestal, then the Poni Island Guardian will challenge you

Tapu Fini Level 60

OT: Yours ID: Yours Ability: Misty Surge Hold Item: None

??? Nature. Date of Receiving Ruins of Hope. Caught at Lv. 60

Nature's Madness Muddy Water Aqua Ring Hydro Pump

Location: Ten Carat Hill

After you have helped capture all the Ultra Beasts, then Looker will return to the motel and exclaim that he saw a mysterious black Pokémon flying over Melemele Island. Anabel reassures him that it was just his imagination as all the known Ultra Beasts have been captured, so he shrugs it off. However, if you go to the Farthest Hollow in Ten Carat Hill, you may find the Legendary Pokémon, Necrozma. While appearances show it is akin to Ultra Beasts, it is not so the Beast Ball will not have additional functionality on it

Necrozma Level 75

OT: Yours ID: Yours Ability: Prism Armor Hold Item: None

??? Nature. Date of Receiving Ten Carat Hill. Caught at Lv. 75

Stealth Rock Iron Defense Wring Out Prismatic Laser