Bake the cake in a round cake pan according to the package directions.
Allow the cake to cool completely, then carefully remove it from the pan.
Using a sharp knife, cut a triangle wedge from one side of the cake. Cut that wedge into two, one smaller than the other. The smaller one will be the ear and the larger one will be the horn.
Arrange the cake pieces on a serving platter or cake board to create the unicorn shape.
Frost the entire cake with white frosting until smooth.
Cover the horn with yellow sprinkles to give it a golden look.
Arrange the rainbow mini marshmallows around the head to create the colorful mane.
Use the black decorating icing to pipe on the closed eye, eyelashes, and mouth.
Slice and serve.