In an anthology I read approximately 1980, various sci fi and fantasy stories, I think it was hardback, I got it from a UK library.
Only one short story I recall (I may have posted this question online a couple of years ago but I'm not sure)
A man in America returns to a small town near a mountain area, with I think an army buddy, I'm not too sure, it could be some adverse person.
They go up into the mountains, protagonist hadn't been for years, he tells his companion about the odd creatures up here, something called a flatter because it could lie spread out very flat and then take it's prey....think like the robot in T2 when it spread out on the checked floor.
Another creature he mentions is a behinder, so called because it, like a bogeyman, is always behind you.
I think the person with him thought he was fooling and started saying loud nasty things about the mountain, then these things appeared and got him. The protagonist was left trembling because he'd finally saw what a behinder looked like, and now he understood why they stayed out of sight.
The name Yando or Yandry is in my mind for the mountain name