Princess Bride Diy Costume . If you prefer romantic comedies, how about princess bride buttercup and westly costumes?! Definitely a great costume choice! The Princess Bride Authentic Buttercup Adult Costume from www.halloweencostumes.com.au Any long sleeved, collared shirt will do. Great red riding costume dress for buttercup from the princess bride. James (10) was westley, eliza (8) was princess buttercup, and brian (3).