Quote Originally Posted by b_jonas View Post
I believe it's a pair of cymbals, that is, a kind of drum made of two metal disks covering each other, where you hit the pair to each other to sound them. You can sometimes see animal toys holding small imitated versions of them, including wind-up or electric ones that hit the cymbals on their own power (and I personally categorically hate powered animal toys). Humans usually hold cymbals with one hand each, but for Princess Celestia, it's more convenient or more dignified to hold them with telekinesis.
It's a clapper hat:



But because ponies don't have hands, it's hooves.