Name me one flying animal with 4 legs besides a flying squirrel.
Flying squirrels don't fly, but they do glide, as do numerous other animals with four legs such as sugar gliders and flying frogs.
Shaymin, as an example of an existing pokemon that contradicts what you are saying, can fly, and isn't based on something like Pegasus, a hippogriff, or griffin. I'm sure there is more to say on this subject, but I won't bother to continue, my point is made :P
For me, the idea of Sylveon being a normal type, despite the similarities between other normal types, the colour scheme, the evolution of Eevee's pertains to them adapting their environment. How would being a normal type be beneficial?
Personally I believe Sylveon is going to be a ground type. Its colours aren't very strong; pigments wouldn't be needed if a large amount of time is going to be spent living underground, naked mole rat. The eyes are dilated, this could be a style, our pupils become expand when there isn't enough light, underground there is no light. The elongated appendages could be feelers, useful for navigation in tunnels, or sensing vibrations when predators are approaching.
The possibility of it being a flying a type isn't farfetch'd, Shaymin can fly despite not having wings.