Feral zombies can be found occasionally in wasteland cities and always in 7 day hordes. The number in the horde will increase each time, so there will be more in a 21 day horde than in a 14 day. Feral zombies are much harder to kill as they have the durability of a bear. Feral Zombies are the only zombie that is supposed to run at all times. unless staggered via damage, or settings are changed when making the world.