‘In that day’ – the day when this prophecy is fulfilled: not necessarily a single day, but just as well, a period in the future – ‘saith the LORD, I will smite every horse with astonishment, and his rider with madness.’ This is about those who are attacking the church.
You may ask, why does Zechariah use such powerful language to describe hostility, persecution, and difficulty? Well, it is appropriate. It is largely symbolic language, but when you read this, it wakes you up to the intensity of the spiritual battle. If you are going to describe the hostility of Satan and his vast number of demons of darkness against Christians and against the Church of Christ; if you are going to describe this in Old Testament language, it is going to be pretty vicious language.