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 that suddenly appeared in the year 2012 at the time of the disaster. They get weakened by the barrier raised around the walls. They get rapidly stronger when they’re more than a kilometer away from the barrier.

Details
The Horsemen are a form of chimera, and were originally created from experiments by the Hyakuya Church. Horsemen are usually large creatures with wings and humanoid-animal like bodies. They prey on humans alone.

Shinoa mentioned them in episode 7 of the anime, saying "Those monsters only attack humans, as if punishing them for multiplying too much."

Story
Yūichirō Hyakuya encountered a Horseman of the Apocalypse during an expedition. Despite being ordered to run, he disobeyed orders and attacked the monster, as a result he gets punished for insubordination.

When arriving at Harajuku station, Shinoa Squad spotted a young girl being chased by a Horseman of the Apocalypse. Kimizuki was the one who killed the Horseman while Yūichirō was unintentionally used as bait.

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.