Horsemen are genetically enhanced experiments created by Jeb Batchelder and the Remedy by using Fang's DNA in Maximum Ride Forever. They were created to repopulate the planet as well as finish off any of the remaining apocalypse survivors (such as the Flock). Dylan became one so he could fake the Flock's deaths and save them from the other Horsemen.
Most Horsemen look like large, muscular men. Their facial appearance remains the same as before their "upgrade", although their general appearance does vary due to the fact that most of the Horsemen are old mutants injected with a serum known as "The Upgrade".
After being injected with the serum, the Horsemen's senses are greatly enhanced, improving their vision and hearing. Their strength is immense, and despite their large and bulky bodies, they are speedy, agile, and have amazing reflexes. In Dylan's case, his wings and sense of location are much stronger than that of the rest of the Flock (even Max's and Fang's), as displayed while hunting them down.