The Littlehorn Massacre occurred at Luna's School for Gifted Unicorns, in Littlehorn Valley. The massacre was caused when a zebra refugee convoy had to pass close to the school. The refugees were mistaken as soldiers by worried Equestrians, resulting in the school firing upon the convoy. The ponies in the school soon realized their mistakes, but casualties had already been sustained by the zebras.
This event lead to the abdication of Princess Celestia from the throne, giving rise to Princess Luna and the Ministry Mares. This was also a turning point for the ongoing conflict, as the Zebras believed Luna to be Nightmare Moon (an agent of the Stars, the mythological enemies of all living things), ending all possibilities of peace between the two nations. Nearly 200 years later, Littlepip would later encounter Princess Celestia who would recount that opening the school in Littlehorn was her entire fault; that there were two other locations she could have chosen at the time, but had settled on Littlehorn because of the nearby Crescent-Moon Canyon as a means of joking that Celestia would be sending all of Luna's Students to the Moon when she would find out about the schools' construction.
In Other Side Stories
Goldenblood and his student Psalm were the only survivors of the massacre. Goldenblood would be stricken with a crippled body for the rest of his life, the pink cloud visibly scarring his coat and damaging his lungs and internal organs, becoming one of the first Canterlot Ghouls in the process.
Sins of an Alicorn
Dinky Doo was one of the teachers that was brought in to work at Luna's School, and was ultimately killed by the Pink Cloud. Her death had deeply moved her mother Ditzy Doo when she heard the news.