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A very popular theory is that Eevee is based on the fennec fox solely because of the ears, when actually, Eevee's bug-eyed, long eared, bushy tail traits was inspired from how red squirrels were drawn in older anime from the 80s and such. This makes sense considering Eevee was designed to be a companion that would sit on your shoulder, and Nishida (designer of Pikachu+Eeveelutions) had stated she was going through a phase where she was obssessed with squirrels, and may have influenced Motofumi Fujiwara when designing this particular pokemon.