Converting a horse costume into a unicorn costume is as simple as adding a horn. Depending on the construction, there are several ways to do this. If the costume has a hood, you can attach the horn to the hood. If the costume has a mask, you can attach the horn to the mask. If the costume does not have a face piece at all, then you can glue the horn to a headband. Make the horn from a styrofoam cone to create a sturdy horn that can be worn on multiple occasions.
Stick a pencil through the top of the styrofoam cone to make a point up to 1 inch tall at the top.
Make a cone shape with the cardstock paper and wrap it around the pencil and styrofoam cone. Glue the paper to the cone and glue the edges together to hold the paper in place. Allow the glue to dry for two hours. Trim any excess paper at the base of the cone.
Wrap three or four pieces of ribbon around the horn and glue in place with a hot glue gun. Make sure to cover the top two inches of the horn completely with the ribbons. You can space the ribbons a couple of inches apart as you spiral down to the base of the horn.
Cut two foam strips, 6 inches by 1 inch. Glue these two pieces crosswise to the base of the horn with hot glue. These strips should match the colour of the mask, headband or hooded shirt.
Glue the strips around a headband, or cut four slits into the top of a hooded shirt and slip the strips through the slits. Glue the ends of the strips together to hold the horn firmly to the headband or to the inside of the hood. If your horse costume has a face mask, then you can also glue the horn to the forehead part of the mask.