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.