The Steady Wing
The cloud-mare's name is Puddle, which tells you everything about her speed and nothing about her heart.
The race runs three times around the palace, through gates of hanging mist. At the starting horn, five court riders shoot away like thrown darts, and Puddle settles into a rolling, unhurried canter, and you understand you have chosen the slowest mount in the entire sky. “Come on, Puddle!” She flicks one ear back at you, kindly, and does not come on.
You're rounding the second tower when it happens. Ahead of you, the youngest court rider — a wren-small girl on a cloud too new for racing — hits the tower's wind-shadow, and her cloud simply unravels, wisping apart like cotton candy in rain. She drops. Riders ahead don't see. The crowd is a mile of noise away.
She has maybe five seconds of sky under her, and you have a slow, steady mare who is already — you can feel it in her shoulders — asking you the question.