Honestly anons, it all comes down to environment, resources, and predation. All of these MG's have very specific genetic traits that are very obviously influenced by where they originated geographically. Magical shit aside, this is just my opinion on the subject
For example, something like the Ushi Oni would more than likely make it's home in a place like Ohio or even Alaska, they like mountains, and would be able to obtain food and water very easily. An Ushi Oni could very easily push a Grisly Bear's shit in, and prey on Elks and Deer with little effort
Hellhounds would probably come from places like Hawaii or other landmasses with Volcanoes based on how much they love extreme heat, even thrive off of it to an extent.
Vamp mosquito comes from Brazil for obvious reasons
Slimes can survive in many environments as long as they have someone or something to feed on
Monkey girls from Africa! Flinging their poo at niggers
Harpies can nest anywhere, and are highly mobile hunters and gathers, just like their irl counter parts. They'd more than likely take to the cities just like real birds
Arachne's would no doubt swarm forests, and hide in everyone's basements. Not to mention all of the natural cave systems through out the US both underground and in mountains
Tentacles would plant themselves in your back yard
Elves would attempt to take over all facets of academia and so forth, trying to blend in with human kind and get all uppity about how smart they know they are.
I think it would be an interesting thing to see, I want to know which ones would stay wild, and which ones would try to conform to this broken shitty mess of a liberal hellhole.