In appearance, the Sugarplum Fairy resembles a preteen ballerina with the mouth of a lamprey. Several tiny rows of sharp teeth take up the majority of her face, and as such she lacks eyes or a nose. She attacks and devours people using her lamprey mouth.
The Sugarplum Fairy is almost conjured when Holden opens and plays a music box with a twirling ballerina in it. As revealed in a Q&A by Drew Goddard, if the song had been allowed to finish, the Sugarplum Fairy would have been summoned to the cabin.
The Sugarplum Fairy is also seen dancing while on tour at Halloween Horror Nights 23 at Universal Studios Florida in 2013.....
Likely also based on lampreys and the common internet photoshop of a lamprey's mouth onto other objects.