Grand Duchess Maria on horseback: 1908.
Grand Duchesses Maria and Tatiana horseback-riding at Livadia, circa 1910.
Olga Alexandrovna remembered that none of her nieces except for Anastasia really liked horseback-riding. They were all good at it, but none of them enjoyed it as a leisure time activity. The older girls seemed to have kept at it because it pleased their favorite aunt.
Source: The Last Grand Duchess: Her Imperial Highness Grand Duchess Olga Alexandrovna by Ian Vorres