Because of the Org's conquests, and their continuous destruction of worlds, the Dimensional Wastelands, along with a plethora of other dimensions, were created. These places are linked to the worlds where the Org and Society have either set up shop, or greatly altered.
Because of this, all sorts of strange creatures, and dangerous mutant and raider gangs have shown up. These monsters are often displaced into other dimensions, and are hostile to any group, Org, Soc, and otherwise. Many have found the ability to travel into dimensions unaffected by the Org's rampage, and thus creating even more chaos.
Face it, people are just gonna go insane, when the world falls away.
Raider Gangs can have limitless origins, but they're mostly hostile, and savage. Most of them are no different than rabid animals. Rabid animals with human intelligence, and weapons.
Either through Org (or some other faction) experiments, human pollution, toxic waste, or naturally evolved, all sorts of dangerous creatures are appearing. Some sentient, some completely wild. But most are extremely dangerous.
(Sources: Anima, Final Fantasy, Gamma World, Supernatural, Warhammer, Warhammer 40,000, Worldshift,)
Bestiarium - A compendeium of various monsters.
Unversed (Presumed Extinct by official records)