The story (like many) was rather obscure but like a few creepypastas became more known due to being read out in the video-sharing site known as Youtube.
The basis of the "Sparkle" myth is that a strange entity can manifest itself within the fictional "world" of the cartoon known as My Little Pony, often disguising itself as Twilight Sparkle but having glowing eyes that betray its inhuman origin.
Although not a deadly entity like many other creepypasta demons it is known to break the fourth wall and deliver lines that are considered threatening and much darker than would normally be allowed in the show itself.
There is also a suggestion that those who see "Sparkle" become insane, either due to the entity itself or due to the obvious perception society would have of them after claiming such things - the story itself highlights the entity's Lovecraftian nature as near the end of the narration "Sparkle" breaks through and seems to speak to the reader.
Unlike many other creepypasta monsters it is left largely to the reader to decide on how dangerous "Sparkle" is and true to the Lovecraftian nature of both the story and the monster the ultimate fate of the reader is also left to the imagination.