I start every costume creation by finding an example that is done often, but not to an extreme level. I saw that the only unicorn costumes online were baggy store bought suits or a horn duct taped to someone’s forehead. I knew I could do better and add an X-Factor like fog snorting out the nose.
Step 1: Foam Foundation
Create your foundational foam structure. I recommend using a combination of the dense core home foundation insulation and expanding foam (low expansion and high expansion). Cut the foam by scoring and breaking. Glue it together with Gorilla glue and definitely use clamps. You can insert pieces of wire at this stage to reenforce joints. Don't rush the gluing process and let it cure for a solid day or two.
You can then use your expanding foam to get a bulk structure for rounded areas.
Step 2: Foam Carving
You want to start from coarse carving to fine finish work. The best way by far to cut foam is by using a hot wire tool. You can buy one online, but they are quite expensive. I recommend buying a $5 hair dryer at a thrift store, get the heating coil wire out, and then clamp onto the wire with a battery charger (car trickle charger worked great). You have to work in a well ventilated area without a doubt.
Once you have it roughed out, move to an electric carving knife which you can find at a thrift store also for about $10.
Then comes the dremel for the concave portions.
Finally I get the details done with a mesh drywall sander and dry wall spackle.
I also attached a photo showing a foam lego head I made last year to show the rough carving.
Step 3: Upholstering
Buy elmers spray adhesive and carefully stretch the fabric over the shape in sections allowing it to dry inbetween areas. Think ahead to where seams will have to be made. For high stress areas you can dip a staple in gorilla glue and press it through the fabric and into the foam.
Step 4: Amazon Shopping and Lighting
You can see from the video that I mounted LEDs and fiber optic lights all through the mane and in the horn to give the magical rainbow effect. If you do the fiber optic hair extensions, you'll want to cut it into multiple layered lengths or you'll just have a hard line of color at the end.
Step 5: Fog, Confetti, and Spandex
I wanted to add the X-factor of snorting fog.
I took a typical can of compressed air. Remove the top trigger portion. Fill the hole where the air typically comes out with hot glue (don't use too much or you'll clog the whole thing). Drill a hole through the side into the path of the air. Insert your the tiny hose tube that comes with the can, hot glue and silicone into place.
You can then take a threaded rod and screw it into the tip that you filled with hot glue as a trigger extension.
I mounted the can of air in the head using a plumbing mount, screws, and gorilla glue.
IMPORTANT: The can must be mounted upside down or at an incline so the liquid gas is shot out.
CAREFUL: If liquid leaks out of the can it will burn any skin it touches. YOU'VE BEEN WARNED!!!
For the body of the costume, I originally got a morph suit and airbrushed on details. After some demo runs I bought some spandex pants to double layer. No one wants a "blow out" when you're dancing your unicorn butt off in front of 1000 people.