The Slender Men are mysterious entities, resembling an unnaturally tall man in a suit, with impossibly long arms and a featureless face. Tendrils have also been known to emerge from their back, and their exact evolutionary lineage is unknown and in fact its visual appearance may be little more than a hallucination or attempt at visual mimicry. Two separate Slender entities are known to exist: The first, known simply as The Slender Man and the second, known as The Operator.
Interestingly, they share numerous traits with the various subtypes of Wendigos although whether this is an indication of them being related to them or if it is simply convergent evolution of some kind is unknown. The Endermen also share similarities, but again, could be convergent evolution.