Zion has bears, Yao Guai are mutant bears, hence Yao Guai in Zion.
However, seeing as how there's like, no radiation in Zion, at all, maybe some un-mutated bears would've been better. Just use the same model with a new texture. It could work. Also, I thought the Yao Guai were called that because of how many Chinese spies were in DC, and the name stuck. How a bunch of isolated tribals would come up with the same name is anyone's guess.
But whatever. Not a big deal.
The fallout would probably have reached Zion from the urban nuclear targets in the region, and the atmospheric FEV from the Glow leak probably contributed to the change.
I have no problem with reusing, but I hate how it feeds the universal wasteland naming we see. It was bad in Fallout 3 when ghouls were called ghouls (even self-describing as ghouls) and Super Mutants were called Super Mutants. It seems like an area so far off wouldn't use the same name for them. I suppose the Brotherhood must have carried over terms like that, but why would the ghouls that they shoot on sight call themselves what they call them? The best hope for Yao Guai in HH making sense naming-wise is imagining that it was a pre-war term that used to describe bears and was all over. Of course this is a little thing, but it does bother me when I actually think about it.
For the name, the Sorrows' ancestors had books and may have coined the term. The Survivalist was also apparently a former U.S. Marine, and may have had a passing knowledge of Chinese, and may have coined the term himself and taught it to the Sorrows in his notes.