Slenderpony's final design; also note the bracelet he is wearing (given to him by Sweetie Belle)
Fluttershy vs Slendermane by Le Fancy Walrus
Slendermare is the spirit of terror. It is a chalk white pony with no eyes, that wears a suit and has spider-like legs coming from it's back.
Slendermane sighting in Pinkie Apple Pie