Context: The informant heard this saying primarily from her mom as a child; similar to “different strokes for different folks,” this phrase is meant to say that there are different people or things suited for different situations. More specifically, as the informant adds, “not all horses are racehorses.”
Analysis: I was confused when I first heard this, but in context it makes sense – racehorses are meant for racecourses, while others are not. It’s an interestingly horse-specific way of acknowledging individual differences.