There are many short horror films you can watch online, but not all of them are chosen to eventually become full length movies. Did you know some pretty good horror films like The Grudge and The Evil Dead started as just a few minutes of an idea in the director's mind? These ideas are often manifested as a baby movie, like Saw 0.5, and you can find many of these horror short films on YouTube.
Horror shorts that later become full-length movies aren't just lucky - they contain something that needs to be grown by someone with a vision. Studios and investors in the film industry saw these shorts and handpicked them out of the sea of horror shorts online to be fully realized. Even Guillermo del Toro has given a short film a hand.
Sam Raimi's 1978 short, Within The Woods, convinced investors he could direct a horror film and they gave him the go ahead to make The Evil Dead. This short also centers around a cabin in the woods, stars Bruce Campbell, and features the wild shooting style and dark humor Raimi is known for.
Macabre film legend Guillermo del Toro noticed Andrés Muschietti's 2008 short Mama and decided to produce the full length version of the same name. Del Toro was happy to help Muschietti's directorial vision in the final film, calling his short "one of the scariest little scenes I have ever seen.”
Before there were a whole bunch of sequels, there was the 2003 version of Saw (aka Saw 0.5), a low budget short made by James Wan and Leigh Whannell to show to investors. It's only nine minutes long, but features the famously terrifying jaw trap which was said to actually be rusty and dangerous, not some fake Hollywood prop.