This is DLC you can play in Taiko Mode.

*Song: "Pokémon: Let's Go, Pikachu! and Pokémon: Let's Go, Eevee!"
Easy: ★2 Normal: ★2 Hard: ★5 Extreme: ★7

More details on the official game page of Taiko no Tatsujin: Drum 'n' Fun! Pokémon: Let's Go, Pikachu! and Pokémon: Let's Go, Eevee! ...