Atsuko Nishida originally designed Pikachu as a squirrel-inspired Electric-type Pokémon.
During Pokémon's first-game development, lead designer Ken Sugimori wanted more cute Pokémon and asked Nishida to help create one. She developed Pikachu's early design using squirrel-like inspiration, including the idea of storing electricity in cheek pouches. Satoshi Tajiri later characterized the species as a mouse Pokémon.
Pikachu's design was finalized by Ken Sugimori, so Nishida and Sugimori are both associated with its creation, but their roles were different. Nishida created the creature, while Sugimori refined the design and produced the promotional artwork used across the original games.
Pikachu evolves into Raichu with a Thunder Stone, while Pichu was added later as Pikachu's pre-evolution in Pokémon Gold and Silver. These related species are a frequent source of confusion when discussing which Pokémon Nishida originally designed.