Princess Ellie's Secret by Diana Kimpton
Princess Ellie is pony-mad! She loves helping out at the royal stables or riding one of her four beautiful ponies in the palace grounds. It's been years since Ellie learned to ride on Shadow, her little Shetland pony. Now her feet touch the ground when she's riding him. When the King sees how ridiculous she looks, he orders Shadow to be sold. But Ellie can't bear to lose him. She's got to think of a secret plan to save Shadow, and time is running out!