The Imps 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.
You thought an imp was a cute little dude in a red suit with a pitchfork. Think again. This Imp heaves balls of fire down your throat and takes several bullets to die. It's time to find a better weapon than a pistol, if you're going to face more than one of these mutants.
With further mutations they now have a sleeker appearance, only having noticeable spikes along its back. These Imps do not have mouths like their classic counterparts (looking a bit like sawcubi). The back of their heads comes to a point.
A more powerful variant with further mutations is known as the Nightmare Imp is also one of the new monsters in the game.
This version of the Imp also features discrete animations for melee attacks versus ranged attacks, similar to the Revenant in other games. This means that unlike other versions of the game, the Imp will not throw a fireball if his melee attack misses the player (since in other versions, the attack type - fireball or melee - is determined by the victim's proximity at the end of the attack animation).