The Tunnelers were primitive underground creatures which only came to the surface after they were alerted by the cathastrophe of the Divide, Having regular old animals become a huge threat because they are no longer actively hunted by humans and occupying more space is a huge deal too, a epidemic or swarm of insects that have naturally evolve to withstand pesticides that are destroying the crops of a community (the ycould even be the tool of another organization), more experimental creatures ala Nightstalkers, introduce the malfunctioning and feral Cyberdogs in masse, animals from a zoo that survived the chaos and bred, there is a lot of shit that can be done without just resorting to mutate every singl thing, or irradiating it.