Have never, to my memory, had a critter come in a dryer vent.

I don't think critters are likly to use the vent as an enterance,,,,,,,but,,,,,,if you hired me to make it as near impossible as I could,I would install a single damper that is held closed under spring pressure until the event calls for it to open. Very simple to supply power from dryer when it is running. Some hvac supply houses stock them but they are mostly used for industrial applications. Graingers has them in thier catalog if you would like to go on line and see how they work.

I have had a bird build a nest in a dryer vent, so I guess other animals might try to get in.

There is a vent cap that has the flapper inside and a large outer cap that includes mesh covering The flapper keeps out drafts and the mesh should keep out critters. However, many of these caps are plastic and I would recommend a metal cap. Squirrels or mice could make quick work of a plastic cap if they started chewing.

Bruce