degradation of Greek cavalry in Hellenistic era

[Astiryu1]

Alexander the Great was often associated with cavalry. The terrain of Greece is largely mountainous and horses are more of a wide open spaces thing. Here in the US the East Coast Native Americans were never horsemen on a wide scale but the Plains and West Coast saw many different tribes with horses. The Spanish helped with the spread of Horse cultures via South America and Mexico. The Greeks had a small Horse following and wasn't well documented (as far as I know) but certain city states were better than others from what I have gathered. I suppose that the most effectual degradation came from the rapid expanse of the Macedonians into Asia Minor and losses/limited breeding opportunities would have taken it's toll. The Arabs later had a great Horse culture that "could" be a remnant of the Macedonian system or breeds but I am no expert on the Subject. Since the Moors had Spain for a time(middle ages) the Horses we have here are probably descendants of the period as the Spanish didn't really stress cavalry before then. Which Greek states were known for their Cavalry? Are there any records of horse trading from northern cultures like the Scythians or other Steppe cultures? After those cultures waned the source of cavalry could have dried up.