If you haven't seen "Cabin In The Woods" yet, then run (don't walk) to the movie theater and see it. [I have always wanted to say that.]
As a major "Buffy, The Vampire Slayer" fan and as a major horror movie fan, I could not wait to see "Cabin In The Woods" and I was not disappointed. The last time I saw a great horror film in the movie theater was when "The Exorcist" was re-released in 2000.
Back to "Cabin In The Woods" - 5 teenagers spend a weekend at a scary cabin in the woods and one thing leads to another (yadda, yadda, yadda) and an evil unicorn is running through the corridors of an office goring people. [It is at the 1:18 mark.]
In my opinion, this is the best unicorn cameo in a movie since Shrek (2001) when the fairy tale creatures are being carted away. Note the screen shot of a knight pushing the unicorn's butt into a paddy wagon.
Hopefully, we won't have to wait another 11 years for a unicorn cameo.