It was generally believed that the design for Pikachu was based on the mouse.
However, Japanese research suggests that Pikachu may have actually been modeled off thecacera pacifica, a species of sea slug found in the Indian and Western Pacific Oceans.
Even to the untrained eye, the similarities between Pikachu and thecacera pacifica are striking: yellow body, black-tipped pointy ears, forced evolution via thunderstone.
In Japan, thecacera pacifica even carries the nickname ‘Pikachu sea slug’ and is the slug of choice among many Japanese Pokémon fans.