Historically, there isn’t always a big difference between a yokai, kami or oni but for the sake of D&D, a new category of neutral leaning fey are all yokai in the lands of Oriental Adventures.
Yokai tend to be mischievous spirits and sometimes outright hateful. Anything with the fey type and a neutral alignment tend to also have the yokai subtype. There is otherwise no consistency between these creatures apart from many of them being immaterial and gaining the following traits – but even these are not consistent considering the sheer number of such creatures.
YOKAI TEMPLATE INFORMATION
This template may be applied to any fey. The original creature retains its statistics except where outlined below. Any fey creature listed in the Monster Manual (or other monster tomes) could be used as a yokai with little modification.
Type. All yokai are fey (yokai) in addition to the base creatures original type.
Alignment. Yokai can be any neutral alignment. Evil yokai become oni. Good yokai become kami.
Language. All yokai speak Common, Kami and Oni in addition to any local dialect of the area they call home.
Damage Resistance Yokai have resistance to bludgeoning, piercing and slashing from nonmagical attacks that aren’t jade
Ethereal Sight. The yokai can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
Incorporeal Movement. The yokai can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
It also gains the following action:
Etherealness. The yokai enters the Ethereal Plane from the Material Plane or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can’t affect or be affected by anything on the other plane.