Only in Ultra Sun & Ultra Moon
Static Encounter: Hau'oli City (After the Hall of Fame, QR-code encounter)
Its base catch rate is 3 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball. For the look, the best colour match is Cherish Ball or Poké Ball.
Magearna in every game · Pokémon Sun, Moon, Ultra Sun & Ultra Moon interactive map (Alola)