Is the world's fastest horse's (Murph, I presume) "little secret" that he's small? Because unless most of the movie is about how Murph is going around in a long trenchcoat, standing on the back of another miniature horse to pretend he's a full size horse, that's not a secret; it's perfectly obvious Murph is small. I don't think he'll be sneaking that past any racing officials. So is the secret that the horse is a donkey? Again, I feel that if Murph were a donkey--as it appears he may be--that would be picked up pretty quick, particularly because, as mentioned, he's also exceptionally small, and looks like a donkey.
The only way I can figure this "little secret" thing would make sense is if the world's fastest horse is an entirely different horse, and it's about how he has to hide his illegitimate, idiot mule-child (Murph) to keep from disgracing his racing legacy. A friend suggested it might also work if it's like a mix of Cars and White Chicks: a story told from the perspective of the racing vehicle--horses in this case--with Murph going incognito to race with larger horses. I'm not sure I buy it, but I'm sure that's better than what it will end up being (probably an hour-and-a-half male-empowerment lesson that size doesn't matter).
Murph Poster [IMPA]