This is one thing that is always a big hit with the kids! Have someone dressed up with a monster or gorilla mask and/or costume jumping around and making monkey noises.
You can also have a cage for your gorilla, here are a few ideas on how to make a cage....
* Use 12 2"x4"'s and build a square cage/box, spray paint the wood black or a flourscent color (if using a black light), then use either long cardboard rolls or thick rope to make the bars and paint them too, this way your gorilla will be able to bend the bars apart, excape and probably chase a few of your guests around!
Feel free to add a few body parts into the cage!
* I use a large metal dog kennel for the cage. It's worth it if you can find one cheap enough.
If you have a monster in a cage, you can try wrapping the outside of the cage with black plastic. This will keep everyone from seeing what is around the corner, help with the suprise attack, and keep extra light out of this sceen. If you like you can have blood dripping down from the black plastic. Throw some bones and body parts into the cage, set a strobe light in front or behind the cage.