A squirrel inspired Pikachu’s original design, according to designer Atsuko Nishida. Although Pikachu is officially classified as a Mouse Pokémon, its early visual concept drew on squirrel features.
Nishida worked at Game Freak and designed Pikachu along with several other Pokémon. She has explained that she was interested in squirrels at the time and used their rounded bodies, movement, and habit of storing food in their cheeks as design inspiration.
That influence helps explain Pikachu’s cheek sacs, which hold electricity rather than food. Satoshi Tajiri later described Pikachu as a type of mouse when shaping the fictional setting, so both facts can appear in discussions of the character’s origin.
Pikachu’s appearance also changed over time. The earliest game design was noticeably rounder and is often nicknamed “Fat Pikachu” by fans. The anime and later games gradually gave Pikachu a slimmer silhouette, while preserving its yellow body, red cheeks, and lightning-bolt tail.