Shaymin
Our first Mythical is thankfully a fairly easy one to seek out, visit the Florio Nature Park at Night and make your way through the course until you reach the flower field at the very end. On your right, you should see Crystabloom which can be hit with an Illumina Orb - but there s actually two Crystabloom s in close proximity. Seek out the one hiding in the blades of flowers and you should trick Mythical Pokémon Shaymin! They re a quick little devil so nab a picture while you can.
After this encounter Shaymin will also appear at the very beginning of Florio Nature Park Day along with Pichu, Grookey and Scorbunny.