The Shotgun Guy makes an appearance in Doom 64.
The Shotgun Guys 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.
Ditto. Much like the Zombiemen, he was once a Marine, always a Marine... Except in this case.... These guys may look like your old buddies, but now they're nothing more bi-pedal maggots... They are much like the Zomebimen, except these guys are meaner, and tougher. These walking shotguns will provide you with a few extra holes if you're not careful.
Interestingly enough, it uses the sprite of the Former Human, but with a different palette applied (white/grey shirt) to change his suit to more like the original Former Human, making both monsters almost impossible to differentiate in the game's darkness. In this game, Former Humans have green pants, while Shotgun Guys have grey ones. They first appear in MAP01: Staging Area.