Nosferatu may have been the very first horror movie. This silent black and white film came out in 1922 and terrorized audiences at the time. The filmmakers had originally wanted to make a Dracula film, but could secure the rights from the Stoker estate, so they made some changes to the story and made it their own. Well, barely. 'Vampire' became 'Nosferatu', 'Count Dracula' became 'Count Orlok', instead of fangs on the sides of his teeth they were his two front teeth, etc. But, the same narrative was left in tact.
I found a really good version of the complete film on Youtube, you can watch it here. It's a good clear print and someone took the time to add the creepy organ music throughout the whole production just like it would have been performed when the film was made.