Adam Clarke on Zechariah 6:6

The black horses - This refers to the second chariot; of the first the angel makes no mention, because the empire designed by it had ceased to exist. This had red horses, to show the cruelty of the Chaldeans towards the Jews, and the carnage they committed in the land of Judea.

The black - Cyrus, at the head of the Persians and Medes, bringing devastation and death among the Chaldeans, called the north in many parts of Scripture.

The white - Alexander, who was splendid in his victories, and mild towards all that he conquered.

The grisled - The Lagidae or Ptolemies, who founded an empire in Egypt; of these some were good, some bad, some despotic, some moderate, some cruel, and some mild, represented by the party-coloured horses.