Little Man on Top (1958)

His grandpa gives the orphan boy Robbie a foal that they call "little brother". The boy loves the animal and wants to train it as a racing horse - but then his grandpa dies. Robbie's sister isn't old enough to care for themselves, so they get a guardian who decides to sell all the horses. Robbie hides his "little brother" at a farm, but it's hard for him to come up with the rent regularly..