The Princess in Black and the Hungry Bunny Horde by Shannon Hale

Princess Magnolia and her unicorn, Frimplepants, are on their way to have brunch with Princess Sneezewort when Princess Magnolia’s glitter-stone ring rings. The monster alarm! After a quick change in the secret cave, Princess Magnolia and Frimplepants are transformed into the Princess in Black and her faithful pony, Blacky. But when they get to the goat pasture, all they can see is a field full of darling little bunnies. Are these bunnies as innocent as they appear?