If you think the costume is magical or creepy...please vote for me in the Halloween Costume Contest: http://tinyurl.com/9nnz7bq.
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.