The Zombiemen of Doom 64 are ressurrected carcasses of those from Doom 1, resurrected by the Mother of All Demons. 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-toing, 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).