Bet on the horse you think will win the race, each horse has stats Min speed (lowest speed it will run when out of stamina) Max Speed (Max speed it will reach until it runs out of stamina) and Guts (Determines how fast the horse will accelorate and how fast it will run out of stamina)
Horses with high max speed and guts will usually win on shorter tracks, and horses with higher min speed and lower guts will have a better chance to win on long races.