THe horse and dragon are included as vehicles and are much easier to win. keep in mind these are not stackable meaning when you win gold and you only get the gold achievement. by far the easiest one I've found was the first race as a dragon- only have to collect 10 (i believe, could be 12) coins and they're all nearby. the dragon challenge is at the bottom of lotso's tree after to run through it
as always if you dislike my guide please tell me why for future guides