Here is the footnote from the Aquinas Study Bible
6:8 pale horse: Rightly is that horse called pale, whose color is that of the dead. For paleness is characteristic of the dead, and in this color is here designated maliciousness and hatred. (Joachim) Or hypocrites are understood. hell followed him: that is, sinners. four parts of the earth: over those who are earthly minded in the world. The sword is fornication. From famine, that is, from a lack of the word of God. And death, that is, with the second death Rev. 2:11 (Ps. Jerome) Our enemy, therefore, destroys the souls of the wicked by beasts, when the carnal motions of the mind are not under the restraints imposed by divine things; but when, mad with the brute impulse, so to speak, of diabolical suggestion, men plunge their lives headlong into the depths of the miserable. (St. Ambrosius Autpertus)
