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 are any creatures that originate beyond the mortal realm. All kami and oni are yokai but not all yokai are kami or oni. Yokai are generally classed as spirits. Any creature from the Monster Manual could potentially be a yokai but they tend to be neutral examples of fey, beasts and plants. All undead are yokai – even evil (and the very rare good) aligned ones are yokai and not oni. Specific examples of yokai here (excluding oni and kami).
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. Add yokai as a subtype.
Alignment. Yokai can be any alignment though they tend to be neutral.
Language. All yokai speak Spirit Tongue in addition to the base creatures language and 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 crystal
Ethereal Sight. The yokai can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
Yokai. As a spirit, yokai have advantage on saving throws against all enchantment effects except those of other yokai. Yokai can sense any passages to Gaki-do and the ethereal plane within one mile.