Griseous Orb (Changes Giratina Altered to Giratina Origin.)
Gracidea (Changes Shaymin into Skye Forme Shaymin)
Arceus Plates (Changes Arceus into a certain type and slightly changes design. Here's the list of Arceus Plates)
Appliances (These are technically not items, but they change Rotom's form to Frost, Heat, Wash, or Mow Rotom.)
Drives (Slightly changes Genesect's appearance and changes the type of it's Techno Blast. They are Burn Drive, Shock Drive, Douse Drive, and Chill Drive.)
I believe that's all. If I missed anything please tell me.
(Therian Forms of Landorus, Tornadus, and Thunderus were introduced in BW2, as were the DNA Splicers, so no Kyurem forms.)