Pestilence seems prone to death by landscape glitches. Care should be taken when traversing a steep mountain, as the horse might randomly die.
As noted in the blood pact description, Pestilence is extremely tough and can withstand more damage than the other horses. It also has infinite stamina. Pestilence is covered with open wounds and has bloody eyes from ocular hemorrhaging, a physical manifestation of disease and plague. It is also surrounded by a sickly green cloud which stuns zombies for a short moment. When standing still Pestilence will bleed out of its eyes and open wounds, leaving small drops of blood on the ground.
The first rider of the four Horsemen of the Apocalypse rode the white horse, actually Biblically referred to as Conquest. It has become Pestilence in popular culture, though the horseman had no traits that would warrant it in the original text. Death was the closest, bringing with it a plague in some interpretations of the Bible.
It is possible for Pestilence to spawn after the player first leaves Blackwater, allowing the player to get early access to one of the Four Horses.
Pestilence has the most health out of all horses in original and Undead Nightmare, which is ironic given its name, nature, and appearance. This is likely due to him technically being a undead horse.
As of June 2014. When playing as Undead Marston. Pestilence will lose his mist, lose his ability to stun undead and steer south. This only happens when summoned via blood pact.