Isn't this a no-brainer? The horse race that wins you Epona is one of the most memorable events in Ocarina of Time, one of those challenges you either totally destroyed or were completely flummoxed by. Many a prodigal gamer was summoned to friends' houses as after school to "just beat that stupid horse race for me."
Getting a horse in Breath of the Wild, unlike in past games, is totally optional. With the ability to climb any surface, you no longer need a horse to jump over a small wall to reach some fenced-off area. But horses are everywhere, which begs the question: Why on earth aren't there any actual horse races?
And no, the devilishly finicky hurdle-jumping challenge at Highland Stable definitely doesn't count.