Decker Horse Show is a jumping championship. It's the main championship in Star Stable: The Winter Rider.
Star Stable: The Winter Rider
Mr. Sands chooses a candidate to represent him in the Decker Horse Show each year. It was Sabine's turn to take part, but Mr. Sands gave the player a chance to compete with her. After the player surpassed Sabine's score, they were chosen to compete in the Decker Horse Show instead.