As part of its ongoing efforts to promote every single Pokémon out there, The Pokémon Company has shared special content dedicated to Eevee:
Did you know? The question of why only Eevee has such unstable genes has still not been solved. Its genes are easily influenced by its surroundings. Even its face starts to look like that of its Trainer.
Source: The Pokémon Company
Pokémon Blog 