Aggressive Mollies?
 

okay so I have a few mollies (7) I have a rubber lipped pleco and I had a rainbow shark.... My rainbow shark is gone!!!!! No he didn't jump out the tank no he isn't in the filter... Do mollies eat other fish?

I don't think they normally do. Unless they are small enough to fit into their mouths then it is possible. Though I have heard of them nibbling on the bodies of already dead fish. Are you sure he isn't hiding? Sometimes fish can be really good at hiding...