“Yaoguai” is a word associated with animal spirits or demons in Chinese tales. That said, Pathfinder’s yaoguai seem far more closely related to the Fallout franchise’s yao guai. These mutated monsters are the result of magically melding creatures together, and they are still tormented by polymorph effects. As such, they are particularly likely to be found near the towers of transmuters and fleshwarpers (who knows?—maybe owlbears are yaoguai that bred true) or in areas of wild magic and magical pollution.
Then again, there’s no reason not to go back to the mythology. Life as a yaoguai might be a cruel fate for oni spirits too clumsy to manifest properly, or for kami spirits who neglect their duties. They might also be expressions of Nature’s rage, mounts for mongrelman heroes, or if (you’re a 3.5 player) a sign of bleed-through from somewhere like the Far Realm.
The transmuter Carlton Griff specialized in creating hybrids—in particular, hawk-headed lions more trainable than natural-born griffons. His yaoguai was a failure he came to regard as a success, and he kept it on the grounds of his manor as a guard. (Repeated polymorph spells trained the beast not to harm him.) In fact, “success” with the yaoguai persuaded him to experiment on his own form, and Griff is now a shrike-headed man with the chest scales of an alligator (treat as a tengu with a +4 natural armor).
A weak and envious spirit, the spirit oni Fourth Lost Hope struggled to even manifest in a mask. After his master was killed and his mask shattered by adventurers, the wisp of a spirit was determined to return and have his vengeance. But once again he was not up for the task and manifested not in a giant’s body, as he had hoped, but in the form of a quill-covered orangutan. Now a mindless horror, Fourth Lost Hope still hunts the adventurers he blames for his condition.
Protean-blooded sorcerers and alchemists set off a chaos bomb in the Wood Quarter. As the warpwave rips through the streets, the chained bear in the bear-bating ring mutates into a yaoguai and begins stalking the streets, slaying innocent and anarchist alike.
—Pathfinder Bestiary 4 284
Where are these D.C. bears the Fallout Wiki references? I’ve lived in this area for 27 of the last 31 years and have never seen a bear. I didn't even see a bear during the four years I went to college in a New England town whose borders were marked with bear crossing signs. Show me these bears, Fallout 3.