Flutter Mane becomes catchable in Pokémon Scarlet and Violet after you have unlocked its spawn location on the Gen 9 map. The new Paradox Misdreavus can be encountered in a specific area hidden in the Paldea region.
Where to find Flutter Mane (Paradox Misdreavus) in Pokémon Scarlet & Violet?
You will need to complete the Starfall Street, Victory Road, and Path of Legends campaigns to unlock Paradox Misdreavus. Flutter Mane only spawns in the Great Crater of Paldea, which becomes accessible in the final Pokémon Scarlet and Violet story campaign. The ancient Misdreavus form becomes available for the first time after you have turned on Research Station No.3 in the crater storyline. After you've unlocked its spawn, you are able to easily track down Flutter Mane in the Area Zero location detailed below.
Where is Flutter Mane's location in Pokémon Scarlet & Violet?
While it's technically possible to catch the ancient version of Misdreavus in the final story, there is a guaranteed location where the Paradox Pokémon can always be caught. Simply head to the Zero Gate fast travel point that is South of Medali City. Flutter Mane's location is next to Research Station No.4, which you can quickly warp to using the green portal in the Zero Gate. After exiting the science lab, you will be able to find groups of Paradox Misdreavus floating around the Paldea Crater caverns. If you don't see the Pokémon right away, just run around the cave's rocky corridors until it appears. Flutter Mane is exclusive to Pokémon Scarlet, so Pokémon Violet users will need to trade with another player to add the ancient Misdreavus to their game.
For more information on Pokémon Scarlet and Violet, check out Where to find Iron Thorns Paradox Tyranitar in Pokémon Scarlet & Violet here on Pro Game Guides.