I've seen both. The Bible seems to support Conquest as the name.
"Then I saw when the Lamb broke one of the seven seals, and I heard one of the four living creatures saying as with a voice of thunder, “Come.” I looked, and behold, a white horse, and he who sat on it had a bow; and a crown was given to him, and he went out conquering and to conquer." Revelation 6:1-2
Pestilence seems to be more prevalent in pop culture.
"Then I saw when the Lamb broke one of the seven seals, and I heard one of the four living creatures saying as with a voice of thunder, “Come.” I looked, and behold, a white horse, and he who sat on it had a bow; and a crown was given to him, and he went out conquering and to conquer." Revelation 6:1-2
Pestilence seems to be more prevalent in pop culture.