Alien
Science-fiction classic Alien is named after the alien that terrorizes a spaceship's crew members as they are floating in space. When the character of Brett is killed, we get to see a little bit of the full grown alien, which is a pivotal moment in the movie.
This scene was edited, however. In the original version, you could fully see the alien in all its terrifying glory as it very graphically murders Brett. Maybe editing these details out was for the best.
The Thing
The ending of The Thing was truly mind-blowing because (spoiler alert!) it ambiguously alludes to the fact that Childs, one of the main characters, could actually be The Thing, the monster in the movie.
An alternate ending, however, shows that The Thing was actually disguising as an innocent dog that just makes his way out through the Antarctic base and nobody realizes.
Mean Girls
Mean Girls is an awesome teen comedy that shows the worst side of high schoolers. When the movie ends, however, the mean girls' clique is dissolved and everyone makes amends. Well, at least you assume they do.
There's a deleted scene that actually shows Cady and Regina making up and burying the hatched. This scene was deleted and replaced by Cady's narrating how things turned out.