Except, it turns out that Pikachu’s visual design wasn’t based on a mouse at all.
Perhaps my strangest experience with learning Japanese has been having to remember two sets of names for Pokémon species.
Aside from those criteria, Nishida was free to do whatever she wanted in terms of design.
Sugimori and Nishida don’t explain why the two-evolution requirement was dropped, but it’s pretty easy to imagine it was a result of Pikachu’s design being unbelievably adorable.