Shemer Arieli, On the Identity of the Horses, Spirits, and Eyes in the Prophecies of Zechariah, Beit Mikra vol. 61 (2016), No. 2, The Bialik Institute
In Zechariah, chapters 1 and 6, horses are used as an entourage for Yahweh and his messengers. They are the eyes in the Lamp Vision in chapter 4. Their joint role is to walk and roam in the world on the Lord's mission, and to report on what is happening in it before the Lord strikes Judah's wreckers (chapters 1; 6) or the ones who are seeking to prevent the construction of the Second Temple (chapter 4). Another purpose of these horses is to lay down the spirit of Yahweh in Eretz Tzafon (6:8).
The background for choosing horses as an image is in the Mesopotamian demonology literature, in a series of magical incantations called "Evil Demons/Spirits" (utukkū lemnūtu), that include the septadic spirits group. Just as in Zechariah (1; 6), they are drawn as "horses who came out of the mountain" and like "chair-bearer of the gods" (and other comparisons). In both genres, the context of their function is combative. The Seven Spirits in Mesopotamia are thought to be equal to the Sibitti gods, which are identified as a reincarnation of the Sumerian Seven Sons of Enmeshara. Some researchers have already identified the Seven Eyes (in the masculine form ""שבעה עיניים) in the Lamp Vision with the Sibitti. The reason why the number of horses is described as four in Zechariah Visions (1; 6) is apparently based on the sovereign processing of the Seven Spirits with the Chariot Vision of Ezekiel (1; 10).