In a large saucepan, heat marshmallows and butter until almost melted. Remove from the heat. Stir in cereal; toss to coat. Press firmly into a greased 15x10x1-in. pan. Cut with a horse-shaped cookie cutter. Remove cutouts to waxed paper; set aside.
In a microwave, melt vanilla chips and 1/2 teaspoon shortening; stir until smooth. Spread over cutouts. Let dry on waxed paper.
In a microwave, melt chocolate chips and remaining shortening; stir until smooth. Place in a heavy-duty plastic bag. Cut a small hole in the corner of the bag; pipe mane, stripes, hooves, etc. on zebras.