>>48831506It happened totally by chance and perfect circumstance, the anime was originally going to use Clefairy instead.
But Pikachu won the favor of Japanese kids because of its design, rarity in Viridian Forest, and allure of being an earlygame electric type (a type you weren't going to see again until Route 10, mind you), all of which made Pikachu an incredibly popular conversation topic.
They went with Pikachu as a result, next thing you know Ishihara was plotting out the marketing craze that would be Pokemania, Pikachu wound up being extremely successful overseas too, and boom, you've got a mascot.
