The four horsemen of the apocalypse are released when the first four seals of the Seven Seals are broken by the Lamb. Revelation 6 has the full story, but the pertinent excerpts are here describing each horseman:
The first horseman: "And I saw, and behold a white horse: and he that sat on him had a bow; and a crown was given unto him: and he went forth conquering, and to conquer."
The second horseman: "And there went out another horse that was red: and power was given to him that sat thereon to take peace from the earth, and that they should kill one another: and there was given unto him a great sword."
The third horseman: "And I beheld, and lo a black horse; and he that sat on him had a pair of balances in his hand.
And I heard a voice in the midst of the four beasts say, A measure of wheat for a penny, and three measures of barley for a penny; and see thou hurt not the oil and the wine.
The fourth horseman: "And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth."
The first horseman is often called Conquest rather then Pestilence. The second horseman is usually known as War. The third horseman is variously known as Famine, Commerce, or Poverty. The fourth is of course Death. These names, with the exception of Death, do not exist in the Bible and were ascribed to them as a shorthand.
Because the description of the first horseman is similar to a description of Christ in another part of the bible, some interpret the first horseman as being Christ. More strongly, others interpret the first horseman as being a false Christ, the Antichrist.
Note the similarity in colors of the horses here with those in Zechariah 6.
Also, the 4 horses could be derived from the horses that drew the chariot of Tiamat, Babylonian goddess who represents the sea and chaos.