You have somehow encountered a wild female Jolteon. This is a pretty rare occurrence since a Jolteon (or any other eeveelution is more likely to be a male than a female.

And I don't think it's possible to encounter an already-evolved Eevee. Oh well. I'm not a dang pokemon professor, I'm the text that tells the story.

The Jolteon simply just stands there, waiting for you to do something before saying; "Jolt! Jolteon!"