degradation of Greek cavalry in Hellenistic era

[PMBardunias]

However, while I am not personally familiar with raising horses, it seems to me that some places have distinct advantages in raising good warhorses when compared to some other places and times.

While I am not an expert on horses, there is a general trend in the domestication of large animals that the first domesticated types are actually smaller than the wild types, then over time the size increases. This is easy to understand if you consider that smaller types are more manageable, and early in the development of animal husbandry this is a big plus. Also, the local varieties of animal in places where they were first domesticated may have been small due to local conditions. For example, wolves at lower latitudes are generally smaller than northern wolves. Modern cattle are still generally smaller than the Aurochs of europe (now extinct).

With horses there seems to be a marked trend towards larger and stronger horses. The horses known as Nisean to the Greeks, "Celestial" horses to the Chinese, were very big for their day, but they were less robust than the big european cold bloods that would either be ridden or mix into horse breeds ridden by knights.