Rebel Infantry: Present throughout the entire series, excluding Metal Slug 5. General Morden's loyal troops are equipped with rifles, pistols, knives, machetes, grenades, bazookas, mines, drop shots, rockets and fire crackers. In graphical terms, they have more sprites and variations than other enemies. We can encounter with them in many aspects: from the future, in space, on water, or can even be prisoners.
Those are some soldiers of the Rebel Army. Here they demonstrate a few of their attacks:
Some soldiers that have been captured or otherwise incapacitated:
The Amadeus Infantry (only appears in the Final Mission of Metal Slug 4):
The Future Rebels (only in Metal Slug 7/XX)
The Ptolemaic Army (only in Metal Slug 5):
Allen O'Neil: Known as "The Immortal Devil Sergeant" of the Rebel Army. He's the most powerful of all soldiers in battle. He usually attack you from the ground, but also can attack you from the air. Equipped with a M60 Heavy Machine Gun, grenades and a knife, he can be a real nightmare.
There are also some other characters based on O'Neil:
Allen O'Neil Jr: The son that follows the example of his father in Metal Slug Advance.
Mecha Allen: An android impersonator of O'Neil. Only in Metal Slug 4.
Sword-Wielders: This type of enemy covers many units with different traits and many sources, but they all have two things in common: They use swords as their main weapon, and have their unique attacks for each group. These are the Arabians, the Pirates, and the Hunters.
Arabians: (عرب) Seen in Metal Slug 2 and Metal Slug X. They are allies of the Rebel Army. Some fight on foot and strike quickly while others ride camels and others throw swords.
Pirates: Seen in Metal Slug 4. They work independently and attack a civilian cargo ship. Some carry guns.
Hunters: Seen in Metal Slug 6 and Metal Slug XX. They attack when the player intrudes on their territory. They wield cleavers, carry dynamite and use some makeshift boats.
Mummies: From Ancient Egypt come the walking corpses known as mummies. Like the zombies, they were disturbed during excavations by the Rebel Army and now rise. In Metal Slug 4, the Amadeus Syndicate controls the curse and uses it to turn the amusement park staff into mummies. They are basic slow-moving mummies, present in Metal Slug 2, Metal Slug X, Metal Slug 3 and Metal Slug 4. Come in a few varieties:
Regular: Most common and less dangerous. Their only attack is a poisonous breath move that transforms you into a mummy. If you are already "cursed" and they breathe on you again, you'll die. Of the 3 types, those are the easiest because their attacks are short and they walk slowly, making them easy enough to avoid.
Drop shot: Distinguished from the previous type by its color (this mummy seems to be off-white, while the previous kind is beige), they rarely move, and their only attack is to drop a cursed orb that turns you into a mummy. Be careful when they appear in groups.
Chariot-spewing: The most difficult mummy to fight. It is in a greenish color and like the previous one, usually doesn't walk. The danger is in its attacks: the chariot they spit up will follow you continually until you destroy it, and will cause instant death upon contact. Even if you destroy the mummy, the chariots will still follow you.
Dog Mummy: A variation of the basic mummy design. Dog-mummies are similar to beige mummies, except that they move faster and jump as well. Their attack consists in a "cursed" bark with the same effect as the beige mummy's breath.
Bats aren't technically mummies, but they can also transform you into a mummy. They appear in Metal Slug 2, Metal Slug X, and Metal Slug 3. Bats carry around a tar containing a cursed liquid that they will dump on you to turn you into a mummy. The bats from Metal Slug 3 hold cauldrons of insta-kill acid instead.
There is also another more peaceful type of bat present in most games that simply act as destructible scenery. Some of them are golden colored and give big points to the player if picked up without killing it.
One of the employees of The Haunted House Egyptian attraction that comes out in Metal Slug 4. It is just a human which will only pretend to attack you. If you attack him, the human will stop acting like a mummy and shiver in fear. But when the real mummies attacks him, he will also become hostile and attack you.
Mummy Cats, unlike Dog Mummies, are female, humanoid mummies with cat ears and tail. They attack by spawning Mummies in front of them. Appear in Metal Slug Attack.
Mummy Warriors are large, humanoid mummies who serve the Pharaoh. They swing a long, flaming torch to burn their enemies. Appear in Metal Slug Attack.
Mutated Soldiers: An experiment created by Rebel Army, possibly on war prisoners. They are mutated creatures that can crawl quickly on the floor and walls. They attack by exploding when near the player. They appear in Metal Slug 2 and Metal Slug X. Very similar to the Hopper Mechas.
Martians: Also known as "Mars People", are cephalopod-like aliens who occasionally ally with the Rebel Army (they provide some of their technology for the creation of a small number of Rebel machines and weapons). They appear in Metal Slug 2, Metal Slug X, Metal Slug 3, Metal Slug 6 and Metal Slug XX. There are three different kinds based on color and weapons:
Gray: The normal infantry of martians.
Brown: One level up. They are more dangerous than grey martians, also more damage resistant.
White: The highest ranked martians. Their advanced attacks in addition with their high damage resistance makes them very dangerous. However, they are seen only a few times, and not in large groups.
The Mars People Rangers are a trio of martians who use special weapons created by the Professor. The Attacker uses a laser gun, the Healer uses a healing staff, and the Tank uses a shield. Present in Metal Slug Attack.
Martians are also present in mini-UFO's that fire single laser shots. The black UFO is more powerful and resistant than grey UFO. Only grey martians and brown martians drive it.
Another type of UFO that doesn't need a driver to fly and attack. They are very fast in their moves and attacks, but they usually fly away after the attack. These UFO's are seen only in Metal Slug 3, Metal Slug 6 and Metal Slug XX.
Note: You can see mini-UFOs in some levels of Metal Slug 1, Metal Slug 2, Metal Slug 3, Metal Slug X, and Metal Slug 6. Just watch the sky. Also one of them appears as a cameo during the final credits of Metal Slug 7/XX.
Chowmein-Conga: Large mutated crabs that appear in Dr. Moreau's Island and in an underground cave full of mutated bugs. They attack with toxic bubbles and their pincers. Only appear in Metal Slug 3.
Ohumein-Conga: They are similar to the previous one except that those are larger, more powerful and more damage resistant. They attack with toxic bubbles and pincer melee attacks.
Huge Locust: Another experiment of Dr. Moreau. Attacks by grabbing and biting the player. Appears only in Metal Slug 3.
Jellyfish: Appear in Metal Slug 3 and Metal Slug 5. Attacks by electrocuting the player. The Metal Slug 5 variant (the one on the right) divides into four smaller jellyfish upon death.
Giant Eels: Another experiment of Dr. Moreau. They don't attack directly but can crush the player against a wall. They appear only in Metal Slug 3.
Flying Killers: These fishes are also an experiment of Dr. Moreau. Their attack consist in jumping out of the water or from a toxic barrel and bite the player. The barrel must be destroyed in order to stop them. Present in Metal Slug 3, Metal Slug 6, and Metal Slug 7/XX.
Zombies: Undead humans that are slow yet deadly in large numbers. They attack by throwing rotten fluids from their bodies. Present in Metal Slug 3 and Metal Slug 4. There are three types:
Normal Zombies: Civilians that have been converted into zombies.
Tar Man: Zombies that are more powerful and damage resistant.
Rebel Zombies: Rebel Soldiers that have been converted into zombies. They attack by leaping at the player.
Sasquatch: Icemen that attack by encasing the player in snow and clubbing them with a bone.
Security Systems: Present in bases and factories, they will attack the player on sight. Appear in Metal Slug 3 and Metal Slug 4. There are two types:
Supervisory Camera: Scans the room and shoots the player with a machine gun.
Patrol Robot: Moves around and attacks the player by shooting a laser with his eye-like front, which is also the only vulnerable area of his body (shooting other parts simply makes bullets bounce away). It needs to charge up before shooting, warning the player of the incoming attack with the flashing red light above it. This one also appears in Metal Slug 3 and Metal Slug 7/XX.
Man Eater: Another experiment of Rebel Army. Moving predatory plant that eats the player. They will sometimes turn into a taller plant that will spit out a seed. The seed will either kill the player on contact or hit the ground and grow into another Man Eater. Present in Metal Slug 3 and Metal Slug 7/XX.
Division 6: These soldiers of the Japanese Army will attack the player with swords, bombs, men-powered tanks and planes with a pulley system. The red-dressed variant will Kamikaze upon defeat as a last try to kill the player. Appears in Metal Slug 3 and Metal Slug Advance.
Maggot: Attacks the player by spewing acid. They automatically die after attacking. Appear in Metal Slug 3, Metal Slug 5, Metal Slug 6, Metal Slug 7/XX and Metal Slug Advance.
Giant Caterpillar: Attacks by ramming the player against the edge of the screen or by spewing acid projectiles from its tongue. Appears in Metal Slug 3.
Snail: Another experiment of Dr. Moreau. Attacks by spitting acid. Appears in Metal Slug 3.
Tiny UFO: Flying drones created by the martians. They attack by firing laser projectiles. They appear in Metal Slug 3. There are two forms:
Type-A: Standard drones that come in large groups. 
Type-B: Resembles the TIE fighter from Star Wars. 
Ring Laser Mecha: A blue robot unit controlled by an artificial brain. Attacks by firing laser rings. The red variant is more powerful. Appears in Metal Slug 3. 
Hopper Mecha: One-eyed Martian drone. Attacks by self-destructing near the player. Appears in Metal Slug 3. It is very similar to Mutant Soldier. 
Mars Mecha: Walking Martian drone that attacks by dropping large bouncing bubble-like charges similar to the Drop Shot's bullets. When destroyed, the body of the robot crashes down, crushing anything beneath it. Appears in Metal Slug 3.
Clones: Cloned versions of the captured Regular Army soldier produced in the Rugname. Appear in Metal Slug 3.
Scientists: Scientists that work at Amadeus's laboratories. They attack by firing a dart with a chemical substance that transforms the player into a chimp. Appear in Metal Slug 4.
Morden Robots: An android impersonating Morden. Attacks the player with a bazooka, then after taking enough damage his head pops out and his body electrocutes himself, trying to kill the player. Appears in Metal Slug 4.
Natives: Jungle tribesmen allied with the Ptolemaic Army. They attack by throwing spears, lit torches and a tomahawk. They can use those tomahawks to protect themselves. Appear in Metal Slug 5 and Metal Slug Advance with different masks.
Invaders: Another alien race that invades Earth. Some explode and release several maggots, while others are gifted with wings and spit fire and others perform rolling attacks that deflect bullet. Appear in Metal Slug 6.
Invader Squad: The most common breed of Invaders. These toad-like alien creatures have features of insectoids such as a rock-hard exoskeleton that protects them from damage. The red variant is more powerful.
Believers: Members of Anastasia IV's cult. Masters use laser guns, Elites use guns that fire stunning spores, Soldiers raise a wall from the ground as a shield. Present in Metal Slug Attack.
Valerian Bears: Enormous, holographic, AI Teddy Bears created by White Baby. Viola uses melee attacks, Rosa throws bombs, and Bubu tosses poison bombs. Some take control of large tanks. Present in Metal Slug Attack.