Monsterween: The Hidebehind
Early colonial loggers in North America knew three things: First, the ancient primordial forests of the new world were creepy as all get out. Second, they kept hearing footsteps behind their own, but when they turned, nothing was ever there. And third, a lot of loggers went missing. As a result, they began to worry that something with preternatural quickness and the ability to hide behind the skinniest sapling was stalking and killing them, and the legend of the Hidebehind was born.
As I've been running a campaign set in a world best described as "Americana Pastiche with Wizards And Golems," I've leaned on a lot of "American monsters." My PCs haven't found their way into any of the truly ancient forests of the setting yet, but I tend to prepare in a round-robin fashion so I have some material to lean on wherever they go.
Certain co-collaborators on this blog ought to ignore the following stat block for fear of spoilers.
The Hidebehind is frightening mainly because it is so unknown. Unravelling that mystery would be quite an accomplishment for a group of heroes. Facts about the Hidebehind's appearance and behavior are up to the DM to decide, changing the creature type, alignment, and languages spoken as appropriate.
Shrouded in Mystery. Many fear the Hidebehind, particularly superstitious lumberers, trappers, and woodsmen. But there is little consensus as to what a Hidebehind actually IS. Some say it is a powerful predatory animal not unlike a bear or a large wolf, but that it walks erect (explaining its footstep pattern) and is unusually tall, thin, and flexible, allowing it to take the shape of any cover no matter how scant. Others say it is a spiritual guardian of the forest, enraged by the trespasses of mortal intruders and their careless exploitation of the local resources. Some say it is an incarnation of the logging camp's own paranoia, feeding on their fear.
Unseen Stalker. The Hidebehind is either very cautious or very selective when it comes to attacking victims. For every missing person attributed to it, there are ten more with a story about being followed by something they could not see, who successfully fled the woods. Perhaps it intentionally disturbs its victims with unsubtle footsteps in order to panic them and wear them down. Maybe it only harms those who are disrespectful to its forests, and considers its invisible presence warning enough. Or perhaps the creature is deaf, and doesn't understand why its prey always seems to know it is there?