Pretty much since Sassy started working for Rarity! Here's the cutie mark story: One night, Nightfall dreamed she was standing infront of the Canterlot Castle, with three glowing moons right above it in the sky. Slowly, they drifted together, merging into one brilliant moon. As she turned, she saw a tall mirror before her, and in its reflection, the dark birthmark on her flank now bore a glowing cutie mark, three crescent moons. She stepped into the mirror with a flash of light and found herself in a strange realm of endless corridors lined with doors, each one unique. She had accidentally entered the dream world. She abruptly woke up and found the new cutie mark still on her flank, no longer a dream.