Way to unlock: After the conversation with Dennett (the Master of Horses quest).
Quest giver: Seanna (M3,1b)
After the conversation with Master Dennett, you will be allowed to mount a horse. Talk to Seanna next to the stables and she will offer you taking a part in a race. Within the allotted time, you need to pass a distance, on horseback, and pass through all of the checkpoints (the vertical wooden poles). To complete this quest, you need to win three races.