The Zombiemen of Doom 64 are resurrected carcasses of those from Doom 1, resurrected by the Mother Demon. They are made from systematically altered dead carnage remade back into corrupted living tissue. The mutations were devastating. The demons have returned stronger and more vicious than before.
Once a marine, always a marine. Except in this case. These guys may look like your old buddies, but now they're nothing more than pistol-toting, bi-pedal maggots. Waste 'em!
In Doom 64, they use a visual appearance that is similar to the Vanilla Doom looks, but instead their rifles look more like shotguns and their suits are colored blue-grey. They first appear in MAP01: Staging Area. Also, like in PlayStation Doom, their gunshots deal somewhat more damage than in previous versions (up to about 21 damage).
Zombiemen, like earlier Doom titles, are the chum of the demonic army. Though their singular attacks are a bit stronger than before, it takes very little to kill them; even the pistol will work fine. Their threat comes from their capacity to attack in groups, made especially troublesome if coupled with Shotgun Guys, as the game's inherent darkness makes differentiating them on sight difficult.