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
You can then use your expanding foam to get a bulk structure for rounded areas.
Step 2: Foam Carving
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.