it's because Eevee's english name was confirmed to be "Eon".... that's when Nintendo changed it.
Remember Seel? Well, nintendo didn't (and still doesn't) want Pokemon to be named directly after animals, so they changed Seel's name to Seel. That's probably why Eevee was changed from Eon to Eevee... Although, despite the fact that an "Eon" isn't really an animal, but a type of animal. This is still the same thing though.
