Vivillion and Flabebe, Floette, and Florges you already know.
Furfrou (grooming it changes its hair style)
Aegislash (form change between shield and blade form based on whether you use offensive or defensive moves.)
Pumpkaboo and Gourgeist (different sizes. Pumpkaboo gets more HP and less speed the larger it is while Gourgeist gets more Attack and less speed the larger it is.)
Meowstic (gender difference)
Pyroar (gender difference)
You also have the old staples such as Rotom. Arceus gets a Fairy form btw.
More might be added later as more info comes in.