The Beast costume is a widely popular men's costume based on the traditional fairy tale character from Beauty and the Beast, originally written by Gabrielle-Suzanne Barbot de Villeneuve in 1740. However it is almost exclusively represented by the intensely popular Disney film rendition.
A good idea for a couple is to dress up as Beast as well as Beauty (also known as Belle in the Disney adaptation). A third person can also join the group as the Disney film's antagonist, Gaston.
Those looking to be more original with their own Beast costume may attempt to use their own imagination to design him or to dress up as one of his various other appearances in media over the years; however in these cases the wearer should expect that many if not most people will question who they are supposed to be.