The Enemies in Duke Nukem Forever range from humanoid to alien, but they all die in a similar way. Currently there are over fifteen different enemies confirmed in DNF, as stated by George Broussard on the 3D Realms Forum. Only confirmed enemies/boss enemies will appear in this section.
Pig Cops are basically large, muscular, anthropomorphic pigs. Most of them are bare chested, but some still wear an EDF uniform, hinting that they were EDF soldiers before they were mutated.
The first type of Pigcop is the classic gun-wielding type. These types of Pig Cops have been seen wielding the M1911 Pistol, Shotgun, Ripper, and the RPG (rarely). The ones holding the Ripper wears a wrecked EDF attire while the other three are shirtless. Sometimes if injured, they will drop their weapons and revert to melee attacks. When they do this, they are exactly the same as the second type as detailed below.
The second type, new to DNF, is the melee type. These types are a lot more animal-like than the classic Pigcop. Instead of shooting the player, they will charge towards the player, sometimes on all-fours, and try to pounce or swipe the player with its claws. These guys are always shirtless and look like the Pistol, Shotgun, and RPG Pigcops.
The third type, also new to DNF, is the Super Pigcop. These substantially bigger than the other two types and also wears wrecked EDF attire. They carry the RPG most of the time and have more health which makes killing them top priority over the others.
Pigcops will sometimes carry a pipe bomb, which they use to flush you out. Normally only the first type of Pig Cops carry these.
The Assault Trooper is a humanoid alien with green armor plates, a laser gun, and a Jetpack. It attacks by quickly firing its AT Laser gun at the player in three round bursts. It can also use a jetpack to fly around if the area permits it. They look very similar to the Assault Captains except they cannot teleport, their armor is green, and they don't have helmets.
Like the original it has red armor, can teleport, and has a jetpack like their Assault Trooper brethren. However this time around the assault captain has armor plating on his legs, shoulders and arms as well as sporting a helmet. The armor plating and the helmet enables him to take a good amount of punishment. In addition instead of carrying the AT Laser as wielded by their Assault Trooper brethren, they carry the more powerful AT Captain Laser which is faster and stronger than the AT version, and has the ability of fully automatic fire. Due to the Assault Captains ability to teleport around they can quickly capture women and almost immediately teleport away before Duke can react.
The Enforcer is a dinosaur-like bipedal monster with a nose ring. It is armed with the Enforcer Gun, which fires homing mini rockets at the player. The Enforcer is a strong enemy when using melee attacks. It is also extremely slower than its agile counter-part from Duke Nukem 3D and lot tougher.
Besides the regular Enforcers, there are ones that are equipped with a large metal shield. The shield must be destroyed before damage to the Enforcer can be done.
The Assault Commander is a flying enemy (fatass blob) that fires rockets from its rectum while constantly moving. In addition, it can use its blades as a weapon by charging towards the player, making them deadly in melee as well as ranged combat. They also have the ability to instantly mutate humans into Pigcops, using the weapon on one of their arms, making the Pigcops your priority before the Commander.
The Octabrain is a floating, medium-sized, octopus-shaped alien with three glowing eyes. One of its attacks consists of it tossing junk scattered throughout levels, such as barrels, at the player. It is also able to fire projectiles at the player from its brain. When stuck with a trip mine it will spin extremely fast and go kamikaze, trying to catch you in both the laser trip mine explosion and its own death-throes explosion.
Its deadliest ability is that it can catch any projectiles fired at it by the player and toss them back, such as pipe bombs and rockets. The pre-release podcast has revealed that they are exploited in at least one boss battle, where Octabrains will appear and will catch any projectiles that are fired at the boss, which is used in the Alien Queen and Octaking boss fight. It kinda looks like he does not have skin on his head, so it shows his brain, but this appears to not be confirmed yet.
The Mothership is a huge city sized flying fortress that has three huge laser guns attached to each side. It is most probably responsible for carrying the majority of the aliens to Earth. It can house a limitless amount of Alien Fighters and Dropships.
A Battlelord is a high-ranking alien commander. It is a large, bipedal lizard with a powerful laser minigun and a built-in mortar launcher. There are two Battlelords in the game, one in Vegas, and the other at the Hoover Dam. The Vegas Battlelord is leading an attack on Duke's Lady Killer Casino, while the Hoover Dam Battlelord, which is much stronger, is protecting the alien "wormhole" portal from EDF assault.
The Alien Queen is a gigantic three-breasted monster that is encountered in the Duke Dome. She was unleashed on the Duke Dome by the aliens, some time during the start of their invasion (according to the construction workers near the Dome). She quickly made the place her nest, creating the Hive. The Alien Queen is another alien-human hybrid like the first Alien Queen killed in Duke Nukem 3D. Her head is very similar to the Cycloid Emperor, her lower body, which stretches through and out the Hive, is that of an Octabrain. Her breasts and fingers on the claws are that of human. It is notable that she apparently has the same energy inside of her body as the Octabrains: upon being killed, it erupts in an explosion. It appears that the Alien Queen is bred to be a quick way to establish a hive, as she was able to create it in about 4-6 hours. Alien Queen is the mother of the Pregnators and the source of all organic alien lifeforms of the Hive.
Octaking is the greatest of all Octabrains, and appears just like his lesser brethren, except he is larger in size and possesses much stronger telekinetic and mental powers. Octaking is first encountered in The Forkstop (Part 1). After defeating all the enemies it will come out of the water and attack the player. Since Duke uses a turret, Octaking is quickly forced to retreat once it has taken enough damage, prompting Duke to say, "I'm not done with you yet, Octapussy!" (a reference to the James Bond villain "Octopussy"). This encounter is just a preview of a true boss fight in Generator Room level.
Later, Duke has a showdown with Octaking in the Generator Room. There, he is aided by three Octabrains, and unleashes all his powers. After being defeated, Octaking suffers a massive damage all around his body and looses an eye. The mortally wounded Octaking tries to generate a massive mental blast, which should destroy both him and Duke, but is stopped in time by more firepower.
The Energy Leech appears to be an alien-breed creature placed underwater to protect the Hoover Dam. This giant leech creature is the last obstacle between Duke and his goal of blowing up the Dam to shut down the alien wormhole. It is encountered in the Blowin' the Dam level, and it appears when Duke is about to place the last wall mine on a gate. The creature is very massive, uses its own head as a hammer, can fire bolts of electricity and attempts to suck Duke inside itself like a vacuum. When weakened by Duke's fire, it is killed off by the wall mine carefully placed inside its gill.
The Worms\ appearance is a long tentacle-like body with hundreds of red eyes at the "head" of the creature. Spikes also protrude around its head and some form a beak shape when its mouth is closed. In a small portion of the leaked gameplay duke can be seen fighting the worm underwater. When its mouth is open thousands of teeth can be seen going down its throat causing it to resemble the Doomsday Machine from Star Trek.
On the surface, the Cycloid Emperor (titled simply Cycloid) appears to be the same as his Duke Nukem 3D counterpart as he still fire missiles. However, he actually has several new attacks and is very robotic in comparison to the original. He is also somewhat larger, standing at about 25 feet tall. This Cycloid is in fact a video game character in a video game based on Duke's adventures in Duke Nukem 3D. For this reason his role isn't quite as significant as his original appearance in Duke Nukem 3D, as his attacks aren't too difficult to avoid.
When he takes enough damage from Duke's Devastator he will collapse; Duke then has to climb on top of him and pull out what appears to be a life support cable on his back. The life support cable presumably keeps him alive as well as powering the exo-skeleton. If Duke doesn't do it in time the Cycloid will stand up again; grab Duke and slam him to the ground (his fifth move) which takes away a lot of health and some of the Cycloids health will now be restored which means Duke will have to give him another beating before making another attempt at pulling out the cable.
Once the cable is pulled out the Cycloid will try to stand up again but will soon collapse and his eye will fall out; which Duke will then kick into a goal post just like he did with the original; however it is an interactive scene as opposed to a cutscene. Furthermore when Duke defeated the original he kicked him in the back of the head to cause the eye to fly out and over the goal post but in the case of this Cycloid the eyes already out and Duke just kicks the eye over the goal post which makes both versions similar but noticeably different. Duke defeats this Cycloid in a video game he is playing.
At the end of the game, Duke encounters the new Cycloid Emperor, the current leader of the alien forces. Duke Nukem hears of him a few times throughout the game, especially from the President, who tried to make peace with aliens through the Cycloid Emperor. He is very similar in appearance to the original Cycloid Emperor and his virtual version. However, in contrast to the former emperor, this one has a black eye instead of red, and is much more cybernetic. The Cycloid Emperor commands the aliens' last stand at Hoover Dam, and is seen two times by Duke while making his way through the dam; including a security monitor. After the dam is finally blown and the alien portal is closed, Duke eventually faces the Cycloid Emperor in Final Battle but not before the Cycloid Emperor who has now had enough of the alliance, murders the president, his bodyguards and an EDF solidier (Who tries to stop the Cycloid Emperor with a shotgun). It's unknown if it was the fact Duke destroyed the dam that provoked the Cycloid Emperor into doing this or quite possibly because he had heard about the presidents orders to launch an ICBM to clear out the remainder of the hostile alien forces as well as leaving Duke to die since the president himself now believed there was no chance of an alliance thanks to Duke Nukem; it should also be noted that shortly before the Cycloid Emperor suddenly emerges in the final battle the EDF solidier that was later killed by the Cycloid Emperor states that they think the Cycloid Emperor had retreated since no one had seen him since the dam's destruction which means at this point the president didn't know the Cycloid Emperor was still in the area. Once the Cycloid Emperor is done killing the president he turns his attention to Duke Nukem. The Cycloid Emperor could only be killed when Duke pulled out three tubes, one on each arm and his back, which appeared to be a life support system.