Horsemen of the Apocalypse

The Horsemen of the Apocalypse, also known as the Four Horsemen of John (ヨハネの四騎士,), are monsters created by the Brotherhood of a Thousand Nights, more specifically, the 6th Trumpet of Seraph of the end.

Their first reveal was at Tokyo's Ueno Zoo in the summer of 2012, the summer prior to the Apocalypse. Kureto Hīragi sent Guren Ichinose's squad to investigate it since the Imperial Demons' discovered that the Thousand Night's were using the Ueno district for an experiment.

Details
"Horsemen only attack humans. Almost as if they were sent to cull the world's bloated human population."

- Shinoa Hīragi

Horsemen are large non-sentient creatures with many wings and legs, although they have not been shown to be able to fly.

They attack to kill any human they notice and take no interest in vampires or animals. This excludes the special horsemen that was summoned by Abaddon to kill the army of vampires and humans.

The Japanese Imperial Demon Army uses a barrier around their city walls to drastically weaken the horsemen when they come within a kilometer of the barrier.

Story
Yūichirō Hyakuya encounters Horsemen of the Apocalypse during his assignment as a private in Shinjuku. During one incident, he attacks and kills one against orders, and it is stated he has done this many times before. As punishment for his insubordination, he is sentenced to attend high school classes until he can make a friend.

When arriving at Harajuku Station, Shinoa Squad spots a young girl being chased by a Horseman of the Apocalypse. She and Mitsuba explain that this is a trap by the vampires, and the child is the bait because she would not be able to survive on her own in this world. When Shiho Kimizuki asks how the vampires are giving orders to the Horsemen, Shinoa explains that Horsemen are non-sentient and only attack any nearby humans. Yu disobeys orders once again and rescues the child by throwing her to his squad members. It is indeed a trap as three vampires take that opportunity to attack him. Kimizuki, Yoichi, and Mitsuba step in the kill the Horseman, and Mitsuba delivers the finishing blow.

When Mikaela Hyakuya arrives at Sannomiya, Kobe, he kills a Horseman of the Apocalypse to save a human girl as she flees for her life. The rescued population of humans gratefully become vampire livestock.

They appear again in a ruined section of Shinjuku when Yoichi manifested Gekkoin to take out three of them from a distance. Later on, as Shinoa Squad drives down the Tomei Expressway on their way to the Ebina Rest Stop, they encounter three Horsemen. Yu jumps out and kills them with one strike when they notice him and leap at him. Shinoa and Mitsuba then prank Yu by leaving him behind in the middle of the road. They let him walk for 10 minutes before they pick him up again.

They don't reappear again until 3 months after the events of Nagoya where horsemen are attacking the village the Shinoa Squad plus Mika and Makoto Narumi are taking refuge in. The source of the Horsemen constantly appearing is by a seraph known as the Sixth Trumpet. When the seraph is cured by Yu and Kimizuki, the Horsemen all around the world vanish.

Trivia

 * The Four Horsemen of the Apocalypse are described in the last book of the New Testament of the Bible, the Book of Revelation. It tells of a book in God's right hand that is sealed with seven seals. The Lamb of God, or Lion of Judah (Jesus Christ), opens the first four of the seven seals, which summons four beings that ride out on white, red, black, and pale horses. The Christian apocalyptic vision is that the four horsemen are to set a divine apocalypse upon the world as harbingers of the Last Judgement.
 * The White Horseman symbolizes Conquest. He carries a bow, and wears a crown.
 * The Red Horseman symbolizes War. He carries a sword which is held upwards.
 * The Black Horseman symbolizes Famine. He carries a pair of scales.
 * The Pale Horseman symbolizes Death. He is the only one given a name in the text. Unlike the first three horsemen, he does not carry a weapon or object, and is instead followed by Hades. (The resting place of the dead).