Zombies are the only enemies in the zombies mode. However, there are sevearl variations, which include:
-Regular Zombies (all maps); Fast Zombies (Verruckt, Call of the Dead); Spetsnaz Zombies (Ascension); Shrieker Zombies (Shangri-La); Napalm Zombies (Shangri-La); Astronaut Zombies (Moon); Crawler Zombies (Kino der Toten, "Five", Moon); Phasing Zombies (Moon)
-Hellhounds (Shi No Numa, Der Riese, Kino der Toten, Moon)
-The Pentagon Thief ("Five"); George A. Romero (Call of the Dead)
-Regular Monkeys (Shangri-La); Space Monkeys (Ascension)
Outbreaks at Nacht der Untoten
The Nacht der Untoten map is a rundown bunker which was used for shipments of classified weaponry by Nazi Germany. Several crates are still left unopened around the map. It is likely that many shipments were filled with Element 115 and, therefore, unearthed millions of dead SS Soldiers.
A group of US Marines were sent in to scavenge from the destroyed bunker and were quite surprised to find windows and doors barracaded. But the even more surprising thing was the amount of dead Nazis trying to eat their flesh.
How did the Ray Gun get to the bunker?
The Ray Gun was most likely shipped to the bunker in order for the Germans to use against the invading US and Russian troopes. But the zombies probably invaded and overpowered the Germans, leaving several Ray Guns left in the bunker for the players to use.
Outbreaks at Verruckt
The zombies at Group 935's "Asylum" (which was really a human testing ground) were risen from the enormous cemetary which lies in the courtyard of the building. There are also dead Germans littering the Asylum grounds, which is likely a result of the burning forest and the attacking Marines.
The Marines (who were not at the Asylum during the outbreak) were killed by anyone left inside, except for Tank Dempsey, who was captured by Dr. Groph and later given to Rictofen and Maxis for experimenting.
How did the Ray Gun get to the Asylum?
The Ray Gun, alog with the perk machines, may have been sent to Verruckt for tests to be done on the subjects. The Ray Gun probably tested resistance against the victims once they drank Juggernog.
Outbreaks at Shi No Numa
An early, unnamed Japanese facility for the Germans to collaborate with the Japanese on certain projects (such as the prototype Wunderwaffe DG, which was tested on unloyal Japanese soldiers), which was built alongside a giant 115 meteor that crashed into the swamp. This meteor's high 155 radiation led to the tested Imperial Army soldiers to rise from the dead.
How did the Wonder Weapons get to the swamp?
The Ray Gun was most likely shipped there from Der Riese or another Nazi headquarters. The same was done for one Wunderwaffe DG-2 to show the final results to the Japanese. Remember that they were allies in WWII, so they had to work together. The perks were also sent from Germany.
Outbreaks at Der Riese
The original zombie outbreak was here, at The Giant. Although there was little to no 115 rocks sitting around the facility, Rictofen and friends had excessive use of 115 in teleporters, weapons, perks, and the Pack-a-Punch Machine, and other tests which were done on unlucky subjects. Also, Fluffy (the first Hellhound) gave birth and the vicious puppies attacked. (Hellhounds only spawn around teleporters.)
How did the Wonder Weapons get to the compund?
All the Wonder Weapons in the map (along with the perks, teleporters, and Pack-a-Punch Machine) were built here.
Outbreaks at Kino der Toten
During a Nazi presentation of teleportation, Wonder Weapons, perk machines and the Pack-a-Punch Machine, the high radiation of 115 brought any nearby dead beings back to life. And due to the teleporter being near, more Hellhounds spawned. Also note that Dr. Friedrich Steiner (from the Black Ops campaign) was presenting and had brought along an enormous sample of Nova 6 gas, turning the zombies into Crawler Zombies.
How did the Wonder Weapons get to the theater?
Steiner had worked with Maxis at Der Riese before and had taken the functional trashed Thundergun without telling Maxis. Steiner then stole the blueprints of the Ray Gun and Monkey Bombs and had put his own name on it for a reshowcasing 20 years later at a popular theater.
He also swiped the perk machines, Pack-a-Punch Machine, and one teleporter from the abandoned Der Riese compund (somehow walking safely past all the zombies without seeing them, either). He was going for fame, but only set a zombie horde on Berlin.
Outbreaks at "Five"
While in a peace meeting, JFK, Robert McNamara, Fidel Castro, and (for some reason) Richard Nixon managed to ignore the sounds of dying scientists and special agents in the surrounding Pentagon area. The zombies then broke in, ready for dinner. They zombies came about by the radiation put out by the teleporters and selective Wonder Weapons.
The reason for the Crawler Zombies is because British scientist Dr. Daniel Clarke had brought over Nova 6 from a collaboration with Dragovich in Germany. Clarke gave the Americans the gas, but was misused during the zombie outbreak and created American Zombie Crawlers.
How did the Wonder Weapons get to The Pentagon?
Several Ray Guns were taken from the bunker at Nacht der Untoten after the investigating Marines were killed. In total, five were stolen. The Ray Guns were about to be put into use by the US Government to defeat the Communists when they received a special visit from the zombie horde.
The Thundergun had come in the mail from Germany 20 years prior. A letter in the box was signed by Tank Dempsey, a Marine who was thought to be dead. He informed the Pentagon that he was being held hostage but could not remember much of anything before his imprisonment. The Pentagon Thief (before his death) was a highly intelligent scientist who believed that he could crack the code to fixing the Thundergun. He could not and he died, making the Thundergun unusable for this level. But, before his death, the soon-to-be Pentagon Thief created the Winter's Howl, which froze its victims.
As for the perks, the US Goverment had not begun developing them yet (the first development was in Moon). The perks are added for the gaming experience as well as the Pack-a-Punch Machine, which has not even been discovered by the Americans yet.
Obviously, the teleporters were incorporated by the American scientists during the 1950s for fast escapes to the inner-labs, which lead to a fail-safe bunker. The teleporters do not run on 115 and therefore do not bring Hellhounds to the scene. Even so, the teleporters function differently from the German ones.
Outbreaks at Ascension
Once completeing his plan to trick Gersch into falling into a black hole, Yuri Kravcheski saw his errors as zombies teleported from the future into the 1960s out of the black hole that Gersch had just created. The zombies (which were dead Cosmonauts, which may have included himself and Harvey Yena, who was nearby) killed Yuri and went for Harvey next.
The space monkeys which Yuri had sent into orbit had died in their space ships and were crashing back to earth due to failed satellite performance due to the scientists at the Ascension Group Cosmodrome dying due to the zombies finally found were Harvey Yena was hiding and killed him.
The only reason the zombies were there in the first place were because Gersch had opened the black hole, which spit out the zombies from the Cosmodrome's future. It's a vicious cycle.
How did the Wonder Weapons get to the Russian Cosmodrome?
Stamin-Up was copied from blueprints that H. Porter managed to steal before he froze to death. The rest of the perks have no known origin. Also, the Pack-a-Punch blueprinits were stolen from Der Riese by Harvey Yena on a mission ordered by Gersch.