Wildmen also known as 'Wildies' are grey people who walk around surviving on trees and water. You cannot fight them in any way. They are useful to you because you can convert them using the Convert spell, but you must do this quickly at the start as the life expectancy is very low. They are terribly weak as well, a single Swarm spell can easily wipe them out and they sometimes accidently fall into the water.
They sometimes get resurrected when a player has had heavy damage well into the game and can reform by using the 'Wildies' that have come back.
They also sometimes get "stuck" between water and land, this can lead to unsuccessful converts.