Yao guai are large, ursine animals that are descended from the bears that lived across America before the war. Due to their zombie-like appearance, yao guai may be the bear equivalent of a ghoul, however unlike human ghouls they are not sterile, as evidenced by the presence of cubs. This is further supported by Ruzka, the only yao guai in Point Lookout, who seems to be a ghoulifed pre-War circus bear judging by the ball found in her lair and the posters around the Pilgrim's Landing boardwalk.
Yao guai are extremely fast and powerful creatures, second only to deathclaws in terms of raw strength and speed. Unlike deathclaws, they are less alert; the player may come in contact with one when it is asleep or laying down. Yao guai do not attack bighorners in Zion Canyon. This may appear uncharacteristic, but is consistent with real predators that do not unnecessarily exhaust their food supply.
Unlike the yao guai found in Fallout 3, the Animal Friend perk does not affect them; they will become hostile towards the player character upon detection.
These creatures are nearly identical to the yao guai found in the Capital Wasteland. They are second to only the deathclaw in terms of raw strength and speed. They are also normally found in groups of two to four.
Giant yao guai are larger versions of yao guai. A giant yao guai can easily kill a low-level player if they're not careful. Early in the Honest Hearts add-on, a giant yao guai attacks and kills a green gecko on top of a rock while the player approaches the Dead Horses camp. A few of these can be found near the Red Gate.
It is stated several times through dialogue with Caesar's Legion members that Legate Lanius was campaigning in Utah. This would mean that yao guai are found elsewhere in Utah.
Follows-Chalk will often make mention of how there couldn't possibly be anything as bad as yao guai in the civilized world, indicating that more dangerous creatures like deathclaws apparently haven't reached Zion.