In Indian folklore, the Yech is a small fairy-like being, the size of a cat, who wears a white shell-shaped cap that renders him invisible. If you are able to secure a Yech cap and put it under a milestone, the Yech will become your servant. Although it is supremely strong, being able to shift cities and mountains, the Yech is unable to shift the milestone without hurting his fingers. A Yech who is not in service is likely to be found leading travellers astray.
John and Caitlin Matthews, The Element Encyclopaedia of Magical Creatures, 2013, Yech, p 530