The ultimate goal for many players is to find and photograph all 63 unique Pokémon in the game. Some are obvious, while others are incredibly well-hidden and require specific item usage or interactions.
Refer to the 'Main Walkthrough' for specific instructions on revealing legendary Pokémon and other hidden creatures. Pay close attention to areas where you can use Pester Balls on suspicious objects or play the Pokémon Flute to wake up sleeping Pokémon. revisit courses after unlocking new items, as this often reveals new Pokémon.