The minks and fishers around where I live (Cayuga Lake in New York) are native. The bigger issue with wildlife around here is invasive species, especially plants. So much privet, bush honeysuckle, buckthorn, and multiflora rose. And many others.
Having hundreds in one area is insane (unless it's for special occasions, like the Baja hammerhead convention). You only see that in video games and horror movies; not in nature.
“Why big fierce animals rare?” Sounds like something one of my ancestors might have asked 30,000 years ago.
Jokes aside, great rec