The Bull Demons 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.
Sorta liked shaved gorillas, except with horns, big heads and lots of teeth. They don't kill easily. Get to close and they'll rip your sorry head off.
It's mutations have, given it a bulkier and more menacing appearance, with black horns, smaller red eyes, and accentuated teeth and claws.
The strategy for this demon is to tear to shreds with your chainsaw.