They're mutated from the black bears that are native to the East Coast. It's
highly unlikely you would have seen an identical mutation in any bears on the opposite coast (there ARE black bears apparently in the region that includes the Mojave, but I don't think it's enough to be a sizeable population). It's
possible for other bears from the Northwest region/Yellowstone region to migrate down, but it's probably not too likely they'll head down towards a desert wasteland. And again, it's not likely they'll mutate to become the Yao Guai.
According to the Nevada Department of Wildlife website....
The American black bear is the only bear species occurring naturally in Nevada. Range includes mountainous areas and foothills of Lake Tahoe, the Sierra Nevada Mountains and nearby mountain ranges in extreme western Nevada.
Nevada's population of black bears is estimated to be around 200 animals. This relatively low number in combination with fragmented black bear habitat has led to the protected status of the animal in Nevada.
And a fun fact from the same website
Presently, the black bear is the only kind of bear in Nevada and California. Until the last century, there were also California grizzly bears. But, now the only California grizzlies found in California are on the state flag.
http://www.ndow.org/wild/animals/facts/black_bear.shtm