It's actually a 'Fantastic Vent' but it is commonly known as a fan, which it indeed is!
They come in lots of flavors from strictly manual to fully automatic with rain sensors and auto opening and closing lids, and are priced according to options.
Another way to go is the Maxx Fan. This is similar to a Fantastic Vent, but comes with an integrated cover that when in the open position, still covers the opening to allow full operation in the rain. The Fantastic Vent will not do this unless an aftermarket cover is added, much to the factory's displeasure.
You can check out the respective web site to see the models available. They are great appliances for ventilating your trailer when no 120VAC is available (both work on 12VDC) and keeping it muck cooler than if you didn't have them.