Pikachu’s original design by Atsuko Nishida was based primarily on a squirrel.
Nishida developed Pikachu and its evolutionary line during Pokémon’s first-generation production at Game Freak. She drew inspiration from squirrels because she liked their movement and appearance. Pikachu’s cheek sacs also reflect squirrel behavior: Nishida connected the idea of storing food in cheeks with the character’s ability to store electricity.
The design was not created in isolation. Ken Sugimori later finalized the artwork and promotional presentation, giving the species a unified appearance with the rest of the original Pokémon. Satoshi Tajiri described Pikachu as a mouse Pokémon in the game world, which is why many fans remember it as mouse-like despite the squirrel-based design inspiration.
Early Pikachu artwork was rounder and heavier than the slimmer version familiar from much of the anime. The character’s visual proportions changed over time, partly because the animated series made a more streamlined design easier to animate. Pikachu eventually became the franchise mascot rather than one of the three selectable starters.