Eevee is one of the most popular Pokémon in the franchise. Between it and its eight evolutions (or "Eeveelutions"), there's plenty to love-- the designs, the versatility, the ability in battle. The franchise didn't initially recognize the Eevee line's potential for long-term success. Now that it does, it presents them as mascots on par with Pikachu, Charizard, Lucario, and Mewtwo.
Source：https://www.cbr.com/pokemon-eevee-day/
