I suppose you mean Sylveon (assuming that we agree Holy / Aether would have nothing to do with the Flying type).
That came because of the name, right? Sylphs having been magical creatures associated with the air. I suppose there is some logic in that, but do note that Sylveon itself isn't as much associated with the 'air' part of 'sylph', as it is with the 'magical creature' part. For example, if we look at its Japanese name, Nymphs have nothing to do with the air. Basically, what we can tell about it (based on its look) is that it's supposed to be a creature inspired by fairies, rather than the wind. Also, do note the word 'sylvan', again, associated with fairy-like creatures, and the various localized names containing either the word 'nymph' or 'fairy'.
But all these are beside the point. Just look at its design; nothing about it says "Flying-type". It's coloured pink (a gross inconsistency for an Eeveelution if it was a Flying-type), none of its unconfirmed moves screams Flying type, and so far it doesn't look to actually be flying (though, surely they could be saving this bit for later). Honestly, I'm surprised fans think Game Freak would design a Flying type Eeveelution like that. It's a nice critter, but it's simply not what a Flying type Eeveelution would look like.
Sure, I suppose there is a possibility it might turn out so. In that case, however, it will be the first Eeveelution whose type you won't know just by looking at it, whose colour does not match that type, and whose conception is only roughly related to it.
