The short film is a right of passage among budding filmmakers but sometimes the shorts they make just to demo their skill end up attracting a lot of attention. Some of the most iconic films in the horror genre started this way before getting the big screen treatment.
Andrés Muschietti who directed the 2013 feature film created the short Mama back in 2008. The short ended up getting the attention of veteran filmmaker Guillermo del Toro who praised the film for it's craftsmanship, ingenuity, and horror, and ended up signing on as a producer on the feature.
The Evil Dead (1981)
The cult classic The Evil Dead was originally filmed as a short titled Within The Woods as an attempted to attract investors for a feature length film. The story is much more simple and has some a few differences like native american artefacts instead of the book of the dead but it even stars Bruce Campbell.
The Grudge (2004)
Most people have seen the American releases of the grudge films which were remakes of two Japanese theatrical films called Ju-on, which in turn were remakes of two made for TV movies. Well even before all of that they were two short films called "Katasumi" and "4444444444." Takashi Shimizu directed every film in the franchise besides the Grudge 3 and actress Takako Fuji portrayed the iconic spirit Kayako Saeki in all except part 3, declining the role since Takashi wasn't involved.
This was another short made to find investors. The film is presented in flashback form and features the "reverse bear trap" scene, with Leigh Whannell playing the part Amanda would later have in the feature while Whannell takes the part of Adam in the full-length movie. The iconic doll even makes an appearance.
Paul Solet wrote and directed the short and feature length versions of this film about a baby that dies but survives, giving it a hankering for flesh and blood. The short film really is just a shortened version of the film and features a much more "zombie like' baby. Another that was filmed in hopes of getting the full length treatment that did just that.
Darkness Falls (2003)
Originally filmed as The Tooth Fairy this short was created by Joe Harris who then adapted his film into Darkness Falls, and though he worked on the screenplay, Jonathan Liebesman was brought in to direct the full length feature about the murderous tooth fairy.
Trick 'r Treat (2009)
Season's Greetings" was the name of this short and it has little to do with the actual film besides being about Sam the films' iconic pumpkin headed trick r treater. The short is animated and while Michael Dougherty was creating it he was cut so some of hisactual blood is included with the in film gore.