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Pikachu's design was popular in Game Freak before the games released, but they think that one of the things that made kids really like it was how it had a fairly low encounter rate (5%) in Viridian Forest and only appeared elsewhere in the Power Plant. There are plenty of kids who just went straight through the forest without seeing it, but other kids would encounter it and then tell their peers that they did. It was the first real elemental monster other than your starter, so it seemed like a special early game bonus Pokémon.
