Doom 64 monsters
Doom 64 includes all of the the enemies from Doom and Doom II. In addition they include three new enemies: Mother Demon, Nightmare Imp, Marine bots (which only appear in the title map). All the sprites have been redone for Doom64
In Doom 64 their physicality is considerably altered; the mechanical body is much bigger, with the brain head ducked down inside the machine itself instead of sitting atop it and the legs being longer and bent at upwards-pointing acute angles. Instead of a single plasma gun, the Doom 64 arachnotrons possess two guns that fire out two blue plasma bolt shots simultaneously, but done so in spurts of five instead of a constant rate of fire, though it will continue firing as long as it can see the player. They first appear in MAP14: Eye of the Storm unless the player goes through MAP30: The Lair and MAP32: Hectic, which contain arachnotrons but chronologically come first.
Baron of Hell
The baron of hell in Doom 64 is given a red-orange colour scheme, rather than green, which is instead used for the hell knight. In Doom 64 barons can also injure and infight hell knights and vice versa.
In Doom 64 the cacodemon underwent a major design change, becoming brown, with a single yellow-green eye, and gaining two arms with broken chains attached, and a rather terrifying face. The body of the monster has animation frames (mostly in the way it swings its arms when in flight), whereas the original Cacodemon floated in a motionless manner. It resembles Doom II's original pain elemental. The nightmare cacodemon variation, introduced in the Doom 64 TC, remained red.
The cyberdemons in Doom 64 have not gone through many changes except that they have darker colored skin, are slightly stronger and much larger. The Doom 64 version of the cyberdemon is notable for having the ability to lead its shots, making it an even more formidable opponent; the cyberdemon's third rocket is usually fired at the player's anticipated position rather than the player's current position. The rockets also leave smoke trails and fly out much faster. The cyberdemons' rockets in this game emerge from the launcher itself instead of the middle. While they have the same death sequence animation as their PC counterpart, for the most part (full-body explosion), it's more elaborate in Doom 64, involving its form stricken with several separate explosions before the body essentially evaporates in a haze of fiery blood, replete with a shot of its skeletal frame vanishing with it.
Cyberdemons are relatively rare throughout the game, appearing chiefly in the secret levels or later in the game.
A special variety of cyberdemon appears on the game's introductory title sequence map. This particular cyberdemon is coded to only ever fire rockets toward the position of the cinematic moving camera and will not infight with other monsters, nor will it react to the player if the game is hacked to allow visiting MAP33. It can be killed, but since it has no death frames it will simply disappear.
Doom 64 includes the demon, occasionally called a "bull demon", giving it a more menacing appearance, with black horns, smaller red eyes, and accentuated teeth and claws.
One quirk of the game is that they can deal and receive missile damage to and from barons of hell, so infighting between the two monsters is possible.
The imp is featured in Doom 64, though with a sleeker appearance, only having noticeable spikes along its back.
A more powerful variant 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).
These variations of the lost souls look somewhat different, are slightly transparent, and there is fire only on the top of their skulls. They are also much more aggressive than their PC counterparts, with faster and more repeated attacks. It is also physically weaker than their PC counterparts and can be felled with a single shotgun blast.
While their function in Doom 64 is unchanged, their appearance is greatly altered. They have less fat and a more bear-like face, are taller and wear a metal body harness. Their fireballs are smaller and appear more like fiery brimstone.
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.
The pain elemental appears in Doom 64 with an altered design. It is colored red (like the original cacodemon while the Doom 64 cacodemon was colored brown and had two arms), has a green eye and in place of its arms are two mouths so it can now spawn two lost souls at the same time. Its back has a large horse-like mane. It gives off a low sinister laugh when it has sighted the player. Unlike other versions of the game, standing at point-blank range to prevent him from spawning lost souls is not a viable option; each lost soul that fails to materialize will cause a significant amount of damage, comparable to an exploding barrel - getting hit by both is roughly the same as being hit by a rocket. (This makes the Pain Elemental very effective at monster infighting as well.) This extends to the lost souls he spawns at death as well.
Former human sergeant
The shotgun guy makes an appearance in Doom 64, but interestingly enough, it uses the sprite of the Zombieman, but with a different palette applied to change his suit to more like the original zombieman, making both monsters almost impossible to differentiate in the game's darkness. They first appear in MAP01: Staging Area.
In Doom 64 specifically, inactive spectres are initially rendered as opaque demons with a green tint, becoming translucent upon detecting the player. Upon death, they revert to an opaque state again. Spectres also appear in the Doom 64 Cast of Characters sequence at the end of the game, unlike the spectre in the Doom II cast sequence.
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).