Mancubus appear in Doom 64. Their function in Doom 64 is unchanged to that in Doom II.
The Mancubus of Doom 64 are the resurrected carcasses of those from Doom 1 (PSX or Saturn, since Mancubi don't appear original Doom), 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.
The only good thing about this fat tub of demon is that he's a nice wide target. Good thing, because it takes a lot of hits to puncture him. He pumps out fire balls like nobody's business.
Mutations have greatly altered their apperance. Their pupil-less eyes have blue sclera instead of green, their bodies have less fat and a more bear-like face, they are taller and wear a metal body harness. Their fireballs are smaller and appear more like fiery brimstone. Their movement style (body bobbing continually to the sides while walking, with both arms rigidly outstretched in front of its body) is very reminiscent of zombies in real-life vintage undead-themed horror movies.
Behind the scenes
This type of Mancubus appears on the cover of an issue of Nintendo Power featuring a spread of Doom 64, including a two-page poster that features it in conjunction with other demons.