There exists in Romania an old fairy tale that describes something similar to the Slender Man. It's date of origin is unknown, however it is known to be very old.

The translation is as follows.

Once upon a time there were twin girls, Stela and Sorina. They were brave little girls, and had no fear of the dark, nor of spiders and other crawling things. Where other young ladies and even young boys would cower, Stela and Sorina would walk with their heads held high. They were good girls, obedient to their mother and father and to the word of God. They were the best children a mother could ask for, and this was their undoing.

One day, Stela and Sorina were out with their mother gathering berries from the forest. Their mother bid them stay close to her, and they listened, as they were good children. The day was bright and clear, and even as they walked closer to the center of the forest the light barely dimmed. It was nearly bright as noon when they found the tall man.

The tall man stood in a clearing, dressed as a nobleman, all in black. Shadows lay over him, dark as a cloudy midnight. He had many arms, all long and boneless as snakes, all sharp as swords, and they writhed like worms on nails. He did not speak, but made his intentions known.

Their mother tried not to listen, but she could no more disobey the tall man than she could forget how to breathe. She walked into the clearing, her daughters shortly behind her. "Stela," she said, "take my knife, and cut a circle on the ground big enough to lie in." Stela, who was not afraid of the tall man, nor afraid of the quiver in her mother's voice, obeyed what her mother said. "Sorina," the mother said, "take the berries and spread them in the circle, and crush them underfoot until the juice stains the earth." Though Sorina wondered why her mother asked her to do such a thing, she obeyed, because she was a good girl.

"

Stela," the mother said, "lie in the circle."

Stela, though she worried she might stain her clothes, did as her mother asked.

"Sorina," the mother said, and bid Sorina cut her sister open with the knife.

Sorina could not; would not.

"Please," her mother said. "If you don't, it will be worse. So much worse."

But Sorina could not, and she threw the knife away and ran home, crying. She hid under her bed, afraid for the first time in her life. She waited until her father came home from the fields, and told him of the terrible thing she had found in the woods. Her father comforted her, and told her she would be safe. He went to the woods, his axe in hand, and as he commanded, she stayed by the hearth, waiting for his return.

After some time she fell asleep. When she woke, it was to the sound of knocking on her door at the darkest hour of the night. "Who is there?" she said.

"It is your father," the knocker said.

"I don't believe you!" said Sorina.

"It is your sister," the knocker said.

"It cannot be!" said Sorina.

"I am your mother," said the knocker, "and I told you it would be worse." And the door, locked tight before her father left, fell open as if it had been left ajar. And her mother stepped in, her sister's head clutched in one bloody hand, her father's in the other.

"Why?" wept Sorina.

"Because," said her mother, "there is no reward for goodness; there is no respite for faith; there is nothing but cold steel teeth and scourging fire for all of us. And it's coming for you now."

And the tall man slid from the fire, and clenched Sorina in his burning embrace. And that was the end of her.

Another story involving the Slender Man comes from a small village near the Retezat Forest in the form of a children's game that has been played since the 1920s. In this game, all of the children form a line, and one is appointed the "leader." The leader walks behind the children's backs and lightly taps one. That one is the "Mare Barbat" (tall man). The children then hold their arms out in front of them as the leader child closes their eyes and walks down the line through their extended arms. The children in line brush the leader lightly with their hands, as if they were the branches of trees. When the leader gets to the "Mare Barbat", that child suddenly grabs them. It usually causes a great fright in the leader, as well as those nearby.

Link:

http://www.slendermanfiles.org/home/romanian-stories