While I didn’t know what it said, I recognized the characters from a half remembered lecture Twilight gave me years ago when she was doing a research project on old pony writing systems for Princess Celestia. From what I could remember, back before the unification and the first Hearth’s Warming, each pony tribe had developed their own writing systems, despite the fact that all three shared a spoken language. These writing systems would gradually die out over the centuries, being replaced by the Universal Equestrian Writing System that everypony uses today. Only Unicorn Script survived to the modern day, and even then it’s only used by a hooffull of Canterlot’s oldest noble houses and a few scholars, and very rarely at that. If what Twilight said was accurate, then most expect it to die out in the next few decades.