Shiny Pokémon, first appearing in Gold Pokémon and Silver, are variants of Pokémon whose colors are different from those of the known species. This type of Pokémon returns in Pokémon Legend: Arceusas well as a specific requirement that grants the player a guaranteed static encounter with a person.
After defeating Kleavor, the player should return to the Galaxy Building and access the Professor’s board. After interacting with the board, they will be prompted to accept Requirement 19, known as “A Strange Ponyta”. This requirement requires the player to search the vicinity for a Ponyta that Professor Laventon perceives may have a different color than a normal Ponyta. The following quest markers will lead the player right to this oddly colored Ponyta, which turns out to be Shiny.
Like any other wild Pokémon in Legend: Arceus, the player will have a chance to fight and capture Shiny Ponyta. Returning to Professor Laventon with Shiny Ponyta will reward the trainer with various berries that will aid the player on their journey. Players will also be able to keep this Shiny Ponyta instead of handing it over to the Professor, giving them a new Sparkling Pokémon for their team.
Shiny Ponyta evolves into Rapidash at level 40, where its bright blue flames are replaced by gray flames as Rapidash gains more powerful features. These Fire-type Pokémon are some of the fastest in the game, and also have powerful moves that can help take down Bug, Grass, Steel, and Ice-type Pokémon.
While this static encounter with a shiny Ponyta may be a one-time request, the player may encounter other Ponytas in Hisui at random as they adventure in the area. The method of hunting Shiny, a major activity in the series, is still being explored.
https://dotesports.com/pokemon/news/how-to-find-and-complete-the-shiny-ponyta-request-in-pokemon-legends-arceus How to find and complete the Shiny Ponyta request in Pokémon Legends: Arceus