Place the pork and onion in a slow cooker. Pour the root beer over the pork, cover and cook on LOW until well-cooked and the pork shred easily, usually 8 to 10 hours (or 4 to 5 hours on HIGH).
Drain well and discard extra root beer. Stir in barbeque sauce. Serve on buns, with coleslaw.
Root Beer gives an interesting sweet and spicy tang to this Southern style sandwich.