Invasive species of beasts were nothing new. This was a major issue in the world that every single race knew. Beyond just the fact that existence linked worlds and different races through dungeons, it also linked the beasts.
This undeniable fact was one that caused many deaths and tragedies in the earliest of times. The humans and other races, had been unable to handle native beasts breaking in to their worlds. That was why every race was the same.
They had cities but could not easily push in to the wilderness of their worlds. It was not just their own natural beasts that lived there. It was the dungeons that brought unknown beasts.