The First Horseman of the Apocalypse, Pestilence, is a mob found randomly in the world.
Pestilence and the white horse both inflict sizable damage. Pestilence inflicts ranged-only damage, but its horse casts poison in addition to tossing the player an unreasonable distance.
Pestilence wears leather armor and the horse wears iron armor. Both are healed by poison. Pestilence is two mobs, but the horse is the toughest of them.