This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Yama-bito (Japanese)
The term yamabito, as understood in Japanese folklore, has come to be applied to a group, some scholars claim, of ancient, marginalized people, dating back to some unknown date during the Jomon period of the history of Japan. Yama-bito appear to be men and women who have reverted or been transformed into an animal- or monster-like state via withdrawal from human society.
The term itself has been translated as "Mountain People", but there is more to it than that. It is from texts recorded by historian Kunio Yanagita that introduced, through their legends and tales, of the concept of being spirited away into Japanese popular culture.