Legendary Pokémon in Pokeland can be obtainable in multiple ways, such as Opening Diamond Capsules, Adv.Capsule Gift, Monster Myst or in Events such as Happy Exchange.

Adv.Capsule Gift is a daily event where every day there are shards of a specific pokémon available. Those shards are obtainable according to the amount of 2488 Diamond Capsules that you open:

Instead of listing all the Legendaries, we are only going to list those who are obtainable in the Happy Exchange only (or other events).
If the pokémon that you're looking for is not in this list, it's because it's obtainable in Monster Myst or Adv.Capsule Gift:

• Armor Mewtwo
• Dark Ho-Oh
• Dark Lugia
• Flare Zyharde
• Hoopa (both forms)
• Master of Illusions
• Necrozma
• Ocean Devil
• Soul Magearna
• Tower Entei - Spell of the Unown
• Ultra Moon Necrozma
• Ultra Sun Necrozma
• Unova Kyurem
• Zacian
• Zamazenta