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The animators had a LOT of trouble keeping Pikachu on-model early on. Pic related is from the original Pikachu vs Raichu episode. In some shots he'd look like this, while in others he'd look more like the "bean" Pikachu. As the show went on, the animators slowly shifted more toward the modern look because they didn't have to constantly cheat to make Pikachu look right in certain poses.