A special treasure.
Something only she, herself had found. It had felt like forever to dig out the shiny rocks that she had found in the swamp, their colours dancing like her own pelt. These were her treasure, and she would hide them, away from her siblings - away from her family.
So, under a tree, away from home, she hid her treasures, away from the world. With her hoof, she had carved a large X in the bark.
X Means Treasure, and it was her little secret.