A lawsuit filed by SCG Power Rangers against a company offering what may be a cheap knockoff, tests the waters. From an IP blog via the ABA Journal, here's some expert commentary:
Are Halloween costumes copyrightable because they are garments, you ask? Although a broad category of creative works are eligible for copyright protection, the Copyright Act…excludes any “useful article” (e.g. clothing) — defined as “an article having an intrinsic utilitarian function that is not merely to portray the appearance of the article or to convey information” — from copyright protection. Thus although garments as a whole are not eligible for copyright protection, the individual design elements — for example a floral graphic applied to the fabric — may be copyrightable. In [Chosun Int’l Inc. v. Chrisha Creations Ltd.], the Second Circuit held that Halloween costumes may be copyrightable if the design elements are separable from the overall function of the costume as clothing.
For even more than you want to know, see our earlier post,"Copyrighting Yoga, Recipes, and Fashion."