Pokemon Ultra Sun & Ultra Moon are getting two brand new Ultra Beasts. UB Burst is exclusive to Ultra Sun and UB Assembly is exclusive to Ultra Moon. Not much else is known about them.
Pokemon Ultra Sun & Ultra Moon will also feature new forms for the legendary pokemon Necrozma, based on the legendaries from the original Pokemon Sun & Moon. The Dusk Form of Lycanroc will get an exclusive Z-move.
If you purchase the virtual console releases of Pokemon Gold and Silver on the 3DS, you can get a Celebi for Pokemon Sun & Moon or Pokemon Ultra Sun & Moon.