Horses were unclean, not to be used by jews.
I don't think horses were unclean animals. But ownership was restricted to the kings and they were restricted in how many they could have.
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it-1 p. 1145 Horse ***Horses are mentioned among the beasts of burden that would be used to transport God’s scattered people to Jerusalem. (Isa 66:20) It is therefore notable that in the first fulfillment of the restoration prophecies, the returning Jews brought back 736 horses.—Ezr 2:1, 66; Ne 7:68.
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it-1 pp. 1144-1145 Horse ***God’s chosen people of ancient times, the Israelites, though, were not to be like the Egyptians and other contemporary nations that considered horses and chariots indispensable to safety and independence. Israel’s kings were forbidden to increase horses for themselves. (De 17:15, 16) Instead of trusting in military might, horses, and chariots, the Israelites were to look to Jehovah for help, and they were never to become fearful of the war equipment of their foes.—De 20:1-4; Ps 20:7; 33:17; Ho 1:7.
King David of Israel was mindful of Jehovah’s prohibition against multiplying horses. In his victory over Hadadezer of Zobah, David could have added many horses to his army, but, instead, he kept only the number he deemed sufficient for his immediate purposes and ordered the remainder to be hamstrung.—2Sa 8:3, 4; 1Ch 18:3, 4; compare Jos 11:6, 9; see HAMSTRING.