Enders are people who are still humans but live like savages. All they do is fight and kill.

Most of their weapons are blunt objects and do not appear to have the ability to work a gun.

They are first seen in Season 3, Episode 2 "A New Mission"

"They are worse than zombies. They are humans, but their brains are fried." - Escorpion

It is possible that general Arthur McCandles and The Gatekeeper soldier that were seen in Full Metal Zombie episode were early stage enders and would eventually turned completely to level of enders that we have seen in show. Both of them had started to lose their senses when Survivor Group met them, but they weren't mentally as far gone as enders are.