I am remodeling my upstairs. I want to remove a wall to enlarge the space,but there is a drain vent pipe in the wall. Can I run it out a side wall. And if so what should I cover it with, so critters don't get in.

You can re-route it to the nearest wall and once you get to the ceiling / attic, reconnect it above. Yes, you could run it outside on the exterior. But this section should be metal where exposed ( a.I.: copper, galvanized, cast iron... ). If it is PVC or ABS you will have to build a chase around it.