I quite like this idea, but my first SCP attempt is in dire need of a ground-up rewrite, and I have another in the wings that I'm still pondering. Right now it's basically traditional creepypasta (I actually managed to creep myself out writing it) and I'm not sure how it could be turned into an effective SCP. If anyone wants it, please lay claim to it, and best of luck.
In a barren mountain range in north western Europe is a secluded cave. Deep inside is a wide chamber that appears to have been used in ancient rituals. The walls of the roughly circular space are densely covered in cave paintings. At first glance, they seem much like those found in other caves in that part of Europe, rendered in red and brown ochres.
Look closely, however, and you will see that the most common figures depicted are not apparently human. They are humanoid, but slightly hunched, and while it is hard to be sure owing to the simplistic style of the art, each figure appears to have two short, thick horns on its head, and a suggestion of a tail.
Look more closely still, and you will find some more traditionally drawn human figures. They are being hunted by the others, run through with spears, battered with stone axes. In some scenes the horned figures appear to be roasting humans over open fires, or eating meat from human-looking limbs.
The paintings are disquieting, but offer no apparent threat. Despite the disturbing decor, a traveller seeking shelter from a blizzard might decide to ignore the paintings and camp here. He may even light a fire.
A well-used firepit lies at the centre of the room, slightly raised on a natural mound. Should a fire be lit there, wavering shadows will be cast onto the painted walls. Many will see their own shape superimposed on those eerie paintings and decide to try their luck outside with the snow. Others will not.
Sometimes, nothing will happen. The lucky traveller will emerge the next morning, haggard from a night filled with half-remembered nightmares, tired but alive.
Other times, shadows will move on those walls, shadows cast by nobody present in the room. The wavering firelight will make them hard to see clearly, and some observers may believe them to be a trick of the light. Watch them closely, though, and you will see hunched figures, perhaps with small horns, and maybe a hint of a stubby tail behind them.
They will gravitate toward the shadows of those who are taking shelter in the cave. They may appear to stand over their sleeping forms. They may appear to be armed with spears or rough axes.
A lone traveller sheltering in the cave will never be seen or heard from again. Their fate will be a mystery.
However, those travelling in a group will be woken by screams in the night. They will look for the source of the screams and find that one of their number is missing, their bedroll empty. If they happen to look at just the right place on the wall, they may see the wavering shadows of someone who is not present in the room, or at least not visible.
The fire will be low and red, so the shadows dim and soft-edged, but it will appear that a struggling form is being speared and clubbed by multiple assailants, and then dragged away, out of the firelight.
The screams will fade slowly, seeming to sink into the rocks themselves.
And is that a new cave painting on the wall the next morning? Surely not - the ochre it is painted in is just as old and dry as all the rest. No, that picture of a man being gutted by horned figures must have always been there.