The Olden Pony is the minor antagonist in My Little Pony: Frindship is Magic. She is the villain of a scary story told by Equestrian foals and fillies. Not much is known about her, other than the fact that she looks like a very old mare wearing ragged clothing, and is looking for her rusty horseshoe. She appears in two of Scootaloo's dreams, along with the Headless Horse.
This version of the Olden Pony was just from Scootaloo's imagination and dreams. If the Olden Pony is an established Equestrian folk tale figure, she might still be thoroughly evil and much more menacing in the commonly told stories.
The powers and even nature of the Olden Pony are unknown, but she is probably a ghost or a revenant, since stories involving restless spirits looking for some property that was taken away from them are very common. Judging by her looks, she may also be a "wicked witch" archetype. An example of this is the folktale Just Delicious in which a women has no liver for her husband so she decides to steal some from the corpse about to be buried. Her husband eats it. At night, the spirit of the women starts chanting "Who has my liver? Who who?" until the women gets so scared that she says "HE HAS IT!". The Olden Pony was likely inspired by folklore like this and the line "YOU DO!" from a story very simillar.
She is voiced by Tabitha St. Germain, the same actress as Granny Smith, who The Olden Pony shares her design with.