Went out, i.e. proceeded further in his march, and fought against them. The horses and chariots, i.e. the men that fought from them, or belonged to them; for so horses and chariots are sometimes taken. See Poole on "1 Samuel 13:5".
Related Commentaries of 1 Kings 20:21
1 Kings 20:21
21 And the king of Israel went out, and smote the horses and chariots, and slew the Syrians with a great slaughter.