This whole book is about the events leading up to a yearly event called the Scorpio Races. The Events Leading Up to the Scorpio Races is too long a title though, so Stiefvater shortened it to The Scorpio Races.
But why Scorpio? Why not the Libra Races? Or the Cusp-of-Aquarius Races? Well, Scorpio is a sign for very focused people, like Sean and Puck. Even though we don't know what their birth signs are, they fit the profile for Scorpios: "Scorpios concern themselves with beginnings and endings, and are unafraid of either; they also travel in a world that is black and white and has little use for gray" (Source). Yep—that describes our two heroes to a T. Both of them have their eyes on the prize: Winning the Scorpio Races. And the villains in the book—Mutt Malvern on the human side, Skata on the horse side—really don't have any redeeming qualities, placing them in a very good vs. evil world.