I was reading the Revelation to John. Particularly the account detailing the first four seals - ie. the four horsemen of the apocalypse. These four horsemen correspond to the first four of seven seals laid out in the book.

So I looked and saw a white horse, and its rider held a bow. And he was given a crown, and he rode out to overcome and conquer.

And when the Lamb opened the second seal, I heard the second living creature say, “Come!”

Then another horse went forth. It was bright red, and its rider was granted permission to take away peace from the earth and to make men slay one another. And he was given a great sword.

And when the Lamb opened the third seal, I heard the third living creature say, “Come!”

Then I looked and saw a black horse, and its rider held in his hand a pair of scales. And I heard what sounded like a voice from among the four living creatures, saying, “A quart of wheat for a denarius, and three quarts of barley for a denarius, and do not harm the oil and wine.”

And when the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, “Come!”

Then I looked and saw a pale green horse. Its rider’s name was Death, and Hades followed close behind. And they were given authority over a fourth of the earth, to kill by sword, by famine, by plague, and by the beasts of the earth.

Rev 6:1-8

This imagery is not unique. As a frame of reference, there is one other place in the Bible that describes horses and riders in this kind of context.

During Israel's captivity in Babylon, Zechariah had a vision:

I looked out into the night and saw a man riding on a red horse. He was standing among the myrtle trees in the hollow, and behind him were red, sorrel, and white horses.  -  Zech 1:8

And what were these horses?

“They are the ones the LORD has sent to patrol the earth.”  -  Zech 1:10

And the outcome?

“We have patrolled the earth, and behold, all the earth is at rest and tranquil.”  -  Zech 1:11

The context of these verses in Zechariah is Israel during its 70 year captivity - in Babylon. Despite the nations of the earth at that time experiencing rest and tranquility, God states:

This is what the LORD of Hosts says: "I am very jealous for Jerusalem and Zion, but I am fiercely angry with the nations that are at ease..."

 "I will return to Jerusalem with mercy, and there My house will be rebuilt, declares the LORD of Hosts..."

"My cities will again overflow with prosperity; the LORD will again comfort Zion and choose Jerusalem.”

Zech 1:15-17

So we have two different accounts describing horsemen with riders sent by the Lord. In the Zechariah account, these horses and their riders have a passive role. They are patrolling and observing. 

However, in the second Revelation account, they take on a very active role releasing conditions on the earth that will ultimately lead up to the Lord's desire for Jerusalem and Zion.