It's like someone had planned a whole bunch of teeny slasher movies, but he didn't have the budget for it so decided to take the all the killers/monsters and put them into a single movie.
Owing to the number of lazy ethnic stereotypes used as monsters I'm going to propose the missing thirteenth monster was all in blackface, wore a battered tophat with white gloves and was called King Cotton.
This sadly meant they had to cut out the scene where he throws a hapless pair of copulating teens into a 'gin.