Yellow - Yellow stuff is bad. Yellow blood is either alien of full of pus. Eggs are also yellow and in a horror movie, eggs are bad. Yellow is also an expression for cowardice. Being a coward is not going to get you through a horror film.
Yeti - The only known surviving yeti is currently living at Animal Kingdom in Orlando. He enjoys terrorising guests all day, but doesn't mean any harm really. His name is Eric.
Youtube - The only things that will survive a nuclear war are cockroaches and Youtube, which will have recorded the entire thing.
Yale Lock - In a horror film, locks do not work. The killer will get in no matter what, but at least you'll feel safer until then.