This page offers a guide on how to complete Request 50: Double the Tails, Double the Fun in Pokémon Legends: Arceus (PLA).
This request is picked up from Netta, who can be found outside the pastures in Jubilife Village. She’ll ask you to show her a Pokémon that has “two tail fins”.
Where to Find a Finneon in ARCEUS
Once you have a Finneon, return to Netta and show her it. She’ll then reward you with x1 Star Piece.