This has become my theory. 'Few reasons why, though you've probably heard these arguments before:


1- Ruzka is a pre-war bear that is somehow alive 200 years after the bombs fell? No way, there's no explanation for that one besides ghoulification.
2- They look like ghouls. Yao Guai have nasty, ripped up, mangled skin and strange tones with only patches of fur. They are gaunt, and have pale, empty eyes like a feral ghoul.



These two factors, to me, just scream "Ghoulified bear".


The only counterargument I can think of, and it's a very good one, is that Yao guai can breed, but ghouls are sterile. This is why I say "of some sort". There seems to be some unknown factor deciding whether humans are capable of becoming ghouls or not, let's say it's a gene. It's possible some bears carried a similar gene or biological trait, too, only one that somehow didn't cause them to go sterile. While these new, immortal, radiation-resistant bears could thrive in the wastes, their unmutated counterparts would suffer, and be out-competed and out-bred by the Yao guai. And BAM! Natural selection leads to a new species.


Thoughts Hypotheses?