Pikachu Was Designed To Be A Squirrel, Not A Mouse
In the Japanese newspaper Yomiuri Shimbun, former Game Freak designer Atsuko Nishida explained the origins of one of the most recognized mascots around the world that she designed.
People have noticed that, over time, Pikachu has definitely thinned out from his original chubby incarnation to one that more closely resembles that of a mouse, which makes sense. Pikachu, since the original Red and Green titles on the Gameboy, has been labeled in the Pokedex as an electric mouse. It turns out this only happened after Nishida originally designed him.
"At that time, I was really into squirrels,” Nishida, told Yomiuri in a post translated by Kotaku, "so I wanted the character to have puffy cheeks. Squirrel tails are cute (so I wanted it to have a tail). However, I wanted the character to have a lightning element, so I made it shaped like lightning."
Nishida explained that the Pokemon's iconic cheeks were based on the way a squirrel stuffed its face with food and its cheeks expanded, thus Pikachu did the same with electricity. Game Freak founder Satoshi Tajiri took the Pikachu name and classified the Pokemon as a mouse type, making it the world famous electric rodent.
It does seem like Game Freak has played with these ideas again, though, between the electric squirrel Pachirisu and the electric mouse Dedenne.