Since this thread isn't about fakemons... I might as well try to stir things up...
Sylveon's type is flying because
A-it's the only type other than normal that's believable,
B-have you seen the diagram?
C- "Sylvan" an air sprite,
D- Because a new, cool looking Eeveelution that [may] naturally learn fly, to show off 3D.
And you can b- about how we've already discussed this, but Sylveon the newest X/Y installment, so it's still what this here thread is for.