In Metal Slug 3, zombies were once regular people and researchers that are transformed by the power of The Ten Commandments of Moses. In Metal Slug 4, the boss of mission 4, Big John, is used by the Amadeus Syndicat to turn everyone in the area into zombies.
Zombies are slow-moving, clumsy enemies, but they can zombify any human, both NPC's and players, almost instantly with their infectious fluids. They are significantly more durable than ordinary humans, with even the weakest able to withstand several rounds of fire before destruction. Fortunately, they are slow-witted, easy to outmaneuver, and often trip when fired at. POW are not infected by any type of zombies' attacks.
If the player is hit by a zombie attack, they will transform into a zombie. As a zombie, the player's character moves much slower, jumps much lower, cannot crouch, and is restricted to their default pistol weapon. Also, another attack from an enemy zombie or the level boss will kill a zombified player. On the other hand, the advantages of zombified player are that the player is immune to nearly all attacks from the Rebels, and that the player's bombs are replaced with a deadly wave of bloody vomit which covers nearly the entire screen. If any enemy is killed using the blood wave, players don't get any bonus points from it. zombified players can transform back to human form by obtaining a first-aid kit; it also wears off on its own after staying as a zombie for a very long period of time.
Infected Survivors make up most of the zombie horde. Slow and individually weak, they can quickly gang up and overwhelm the player. Common zombies trip over the most often.
Man: This one will vomit infectious fluid into cupped hands, and then hurl it at its target.
Woman: This zombie will vomit infectious fluid directly at its target.
Young Man: This zombie squirts a blob of infectious fluid from his brain cavity by squeezing its head. It also moves faster than the others common zombies, mostly likely due to its young age.
Fattish Man: This one can launch infectious fluid at its target by opening its ribcage. It takes more damage than the other kinds of zombies to kill it, and has better resistance against grenades than the others..
Scientist: This zombie uses it's intestine as a kind of "hose" in an attempt to spray zombifying liquid on the player. When killed, a first-aid kit will be left behind. It has slightly less health than the other common zombies.
Note: All common zombies act the same, the only difference is their health, speed and toughness.
Uncommon zombies are more dangerous and have unique attributes. They are typically found in specific areas of the level.
Rebel Soldier: This zombie is faster than the others, likely because of they military conditioning. They are encountered near the end of the level, close to the camp where the outbreak probably originated from. As with the other zombies, it is far more durable than ordinary Rebels, requiring nearly a dozen bullets to take it down. They are also unique in that they will leap at the player rather than attacking from a distance, and explode upon contact with the ground or the player. They also do not trip as often as the others Zombies.
Tar Man: The most dangerous of the bunch. This one is just slightly slower than the Rebel. When in front of its target, it attacks by throwing a spread of goo. If they are on a platform directly above their target, they will drop down in an attempt to land on their target in a similar manner to the Rebel. This zombie rarely trips, There is also an exclusive Red version of Tar Man which appears in Metal Slug Defense and Metal Slug Attack.
- Zombies were probably based on the classic "Romero" Zombies as seen in films like Dawn of the Dead. Interestingly, unlike zombies with which most people are familiar, zombies in Metal Slug do not attack by biting.
- While a Zombie, if the player gets hit by another zombie or by the boss, the player will die just like how the other zombies do when killed.
- In Metal Slug Defense, unlike their arcade counterpart, zombies' infectious fluid does not transform enemies into zombie. Instead, they only damage enemies.
- The Scientist zombie has been added later in a Metal Slug Defense's update, and so was the Young Man Zombie.
- The Fattish Man zombie is sometimes referred to as Elderly Man. 
- Uncommon zombies don't show any transformation (From human to zombie) sequence like Common ones.
- Zombies are also used (possible legally) by Armor Games to make ZS & ZS: Outbreak, both of which are based on Metal Slug with Ptolemaic Special Force as player character.
|Human||Rebel Infantry | Arabian Infantry | Japanese Infantry | Pirates | Scientists | Ptolemaic Infantry | Natives | Hunters | Future Rebels|
|Undead||Mummies (Mummy Generator) | Dog Mummy | Mutated Soldiers | Zombies | Tar Man | Clones|
|Mutant||Bat | Chowmein-Conga | Ohumein-Conga | Enormous Moray | Jellyfish | Flying Killer | Man Eater | Huge Locust | Sasquatch | Maggot | Big Snail | Giant Caterpillar | Squid | Huge Ivy|
|Alien||Monoeye | Ring Laser Mecha | Hopper Mecha | Mars Mecha | Mars People | Big Eye | UFO | Mini-UFO | Invader Squad | Winged Invader | Flying Parasite | Hunter Walker | Hunter Lord | Scavenger | Crab-Tank | Vanguard | Smasher | Invader UFO | Indestructible UFO | Flying Core|