They are said to take the shape of giant snakes, sometimes with a Human head. Other times they appear as misty ghost-like humanoids. They may keep their snake pupils and forked tongues and share some things in common with their original reptilian form, like the need to use external warmth and the habit to hivernate in winter/cold temperatures.
Secretive and mysterious, when they appear at all, they seem quiet and reserved, perhaps shy.
Not much is known about the elusive Hebi Yōkai. It seems to be one of the rarest, most mysterious types of non-ghost Yōkai existing in the World.