Preheat oven to 350°F. Prepare 9-inch round pan and bake cake per box directions (usually 25–30 minutes). Cool completely.
Tint half the frosting pink and half brown. Reserve 1/4 cup white for highlights.
Trim top slightly so cake sits flat. Spread brown frosting over lower half of cake for the face; pink on top for the ponytail area.
Press mini marshmallows along the crown where the mane will start. Layer candy pieces over marshmallows to form a colorful ponytail.
Melt brown candy melts and pipe a small ear shape on parchment; chill until set and place on cake.
Use black icing to draw eyes and nostrils. Add white frosting dots for eye shine.
Chill 20 minutes to set candies before serving.