Missing Areas

Metal Slug X has a ludicrously huge level select in contrast to 2. There's around seventy levels! That's a lot of unused areas, though a lot of them are just warps to certain parts in the levels... though that may imply some interesting design changes were once planned for the game. Read on!
Huge thanks to candy76041820 for the name translations.

Thankfully, Metal Slug X's test stages are somewhat more interesting than Metal Slug 2’s.

ANDY THE WIZARD is Mission 1 with a variety of items to get, including a prisoner who gives you an Armour Piercer, another Armour Piercer, a Shotgun, a Rocket Launcher, a Flame Shot, a Laser Gun, an Enemy Chaser, and a Girida-O.

MEEHER is the beginning of Mission 5, but with a Camel Slug and a rocket that’s flying backwards at the beginning, and when you reach the other side of the screen, several mummy dogs and a runaway camel appear.

YOKO-SHU is Mission 4 with various random things appearing in it, such as the credits, two relaxing soldiers, some Metal Slug 1 enemies (mainly just cannons and outposts).

ナミひら is a mostly empty version of the second part of Mission 6, with the only things in it being an unlimited number of Slugnoids at the beginning, and several soldiers along the way.

アリタ is Mission 1 with sword throwing arabs instead of rebel soldiers, and along the way a line of flashing tank shells appears, as well as a square made out of coins and near the end items start being thrown out of your character.

NISHINO is Mission 4, and with a variety of items to grab. There’s a Shotgun, pulse Shotgun, Super Grenade, Iron Lizard, Laser Gun, pulse Laser Gun, Drop Shot, pulse Rocket Launcher, Fire Bomb, pulse Heavy Machine Gun, and a Rocket Launcher. There are also a large amount of enemies, including some from Metal Slug 3!

NAKATSUKA is the first mission, with no objects or music until you reach where the three towers are, where a helicopter come down, and you fight General Morden like you do in Metal Slug 1. After you beat him, he falls out of the helicopter, and the mission is completed.

TAGUCHI is an empty version of Mission 2’s third section, only occupied by a Slug Flyer, two soldiers, and an endless amount of crab items from Metal Slug 3. Near the end, an endless number of coins and mummy jars fly out to your character’s left.

There are no unused stages in Metal Slug 2, disappointingly, but Metal Slug X has a whopping number of 58 unused areas! And even better, is that they’re actually interesting!
Quite a few levels are from different games, so if there are two screenshots, the one on the left is from Metal Slug X, while the right image will be from the game the level appears in.

There are a large number of selections that are just warps to partway through a level; their names appear to suggest they were planned as alternate routes in not only Metal Slug X's stages, but those of the original Metal Slug! Although the warp rooms clearly state where you would have exited from, there's only a few concrete clues to where you would have entered them from.

Metal Slug X / Metal Slug 3

1:2

This is an area from Metal Slug 3 (4:4 2-1, to be precise), but alas, there’s no objects, nor an exit. It appears that when it was used in Metal Slug 3, they mirrored the room, added in the staircase and removed the background from the windows.

The exit puts you at this window before the three cannon towers. I can't peg where the entrance might be with certainty.

1:4

This is the desert area, but at morning, just like Metal Slug 2. Unlike Metal Slug 2 is that it goes on seemingly forever, and there’s no scenery or anything interactive. You don’t leave footprints in the sand, but there are some mighty fine effects on it, like sand being blown about and haziness and stuff. It’s cool.

If you could exit, you would have warped in before the truck full of swordsmen.

Metal Slug X / Metal Slug 3

2:4

Another MS3 level (4:C 4-2), this one is apparently endless, and also has no objects. They changed the pillar slightly, and pretty much reworked the background from scratch.

The supposed exit would've put you in the ruins after the miners are scared away, suggesting the tunnel would've led you here.

Metal Slug X / Metal Slug 3

2:6

This is the spiked ceiling part from Metal Slug 3(4:F 4-5), but the ceiling doesn’t lower, nor are there any obstacles. The area was merely recoloured to look more dark and nasty, but the ceiling was given a massive spit and polish.

The exit doesn't work, but it would deposit you after the mummy generators.

Metal Slug X / Metal Slug 3

2:8

The sand-shifting slope from Metal Slug 3 is here (4:E 4-4), but of course, there’s no obstacles. There’s some very minor edits:The whole room has been recoloured (obviously), but the MS3 version has alternate or less details:

• The MSX version has a block nearest the bottom with an eye on it, but in MS3 it’s merely a circle.

• The block next to that has a duck in it, but the version in the finished product is a fancy shape, possibly a flower.

• The block in the bottom corner has three symbols on it, but when brought into MS3, it lost the third symbol.

• The block above that one has what looks like the German comic character Werner in a Batman outfit, but it’s more like a scribble of lines in MS3.

The block to the left of that has a squiggly line and a dot in MSX, but it’s changed to a squiggly line with a dot on the end in MS3.

• The stone head in the background looks content and happy in MSX, but looks like a frightened Chris Tucker in MS3.

• A block above that head has another squiggly line + dot design in MSX, but was changed into a circle or eye in the sequel.

• There’s a block further to the left of that one; it has what looks like a question mark, or a walking stick on it, but it was mostly erased in MS3.

They’re not major differences, but it’s interesting to see what they changed.

The exit deposits you before the mummy dogs. I can't imagine these exits were given much thought.

Metal Slug X / Metal Slug 3

2:A

Another MS3 level (4:D 4-3), this is the area with tons of items, but the door opens to reveal a bunch of that crazy exploding moths. The only differences appear to be the inclusion of a door and obstacles, and the area made less colourful.
The exit would put you a few steps forward from the last one.

Metal Slug X / Metal Slug 3

2:C

An early version of the stone lift (4:10 4-6), seemingly endless. All they did was recolour it.
This would drop you off before the stage boss if it ended.

3:2

This appears to be remnants of the flying part of Metal Slug 3 (5:2 1-1); once you reach the sky part, your character dies for no apparent reason, and an explosion occurs at the top left corner. Strange.
Its alleged exit would put you at the mid-stage boss.

4:2

It’s just like the sewer areas from Mission 5, but there’s no obstacles, and you can only go so far before an invisible wall blocks your path.
It's meant to exit you before the UFOs appear.

Metal Slug X / Metal Slug 3

4:4

This is the area where you gather batteries for the exit from Metal Slug 3 (4:B 4-1). There’s quite a few changes, such as a computer terminal in MSX replaced with boxes, the platforms risen, and boxes given, taken, and rearranged. The grime in the background was redrawn, and a few pipes were added in.
The exit is identical to the last one.

Metal Slug X / Metal Slug 3

5:4

This is the mutated bug area from Metal Slug 3’s fourth mission (4:8 3-1). Pretty much the entire place was redone, with tons more detail added in.
In MS3, the area changes flashes slowly and gradually, but this is a lot more blatant about it, making the flashing fast and obvious.
Yeah, that's the most detail I can go into. Have a map!
There is no exit warp associated with this room.

Metal Slug X / Metal Slug 3

6:4

The underwater path from Mission 1, Metal Slug 3, complete with Slug Mariner! (1:2 2-1) There doesn’t seem to be much changed in graphics, they just added in a few things and changed the colours of when you drive closer to the bottom. They added in a few “balconies” for prisoners and boxes, and additional details were added to the jellyfish caves and eel cages.
The level ends once you reach the bottom, where you’ll die if you drive to the bottom.
As for the Slug Mariner, it seems rather complete, aside from a few things. It’s gun doesn’t shoot the “floaty” bullets, and goes in a straight line just like the turrets of the Metal Slug and Slug Copter. It’s bomb appears complete, but since there’s no solid ground here, you can’t see what it’s explosions look like; it also has the wrong word in the HUD (“cannon” instead of “missile”).
However, you cannot exit the vehicle, nor can you Slug Charge with it. When it explodes, instead of your character playing the drown animation, they fly out as if above water, even though the drowning animation is in the game.

The exit would put you in the final stretch of the base's interior.

7:1

Does this look familiar? It should, as it’s the first mission from Metal Slug! Besides missing all its enemies and obstacles, it's otherwise somewhat operational with the reflections in the water, the stone traps, even the little fish you can shoot the flesh off of! It also has a Slug Flyer parked right at the beginning, which is great.
The only notable quirks are the camera is positioned lower than usual, the soldiers that emerge from the huts can only run in place, and flying the Slug Flyer through the Middleson's hull triggers the flashing effect normally used for weapons fire.

Metal Slug X / Metal Slug 3

7:2

Another MS3 level, this is also from Mission 1, and is the upper path. (1:4 3-1) The background was made gloomier, and some dead fish were added in MS3.

Going by the level's name, this was meant to be entered and exited from the wrecked submarine at the start of the stage.

Metal Slug X / Metal Slug 3

7:4

This is yet another part of Mission 4 from Metal Slug 3, a part of the upper path. (4:2 1-2) Apart from the typical design changes (the MSX version is unfinished after all), the only change appears to be the removal of the trees and lowering of the background.
This would exit you before the stage boss.

Metal Slug X / Metal Slug 3

7:6

Aha, the samurai caves! (4:7 2-4) These are from MS3, and there are no major differences. Slight colour changes, minor additions, like that rock there.
The exit is the same as last time.

7:8

Starts you a little more into the samurai cave.
The exit is the same as last time.

8:1

The second mission from the first Metal Slug! It gives you access to both the Slug Flyer and, shockingly, the Slug Copter, which wouldn't debut until the next game! There's a few minor differences with it, though: Most of its sounds are missing, it's vulcan cannon changes colour when facing certain directions and fires the same bullets as the Slug Flyer, rather than its oval ones, and its bombs fall straight down instead of at an angle.

The level is largely vacant of obstacles and enemies, though some soldiers rolling snowballs (from MS1's third mission) appear on the bridge. There are soldiers in rubber rings with bazookas under the bridge as expected, though they can't destroy the bridge; the fourth one is notable because it only fires diagonally, using an unused animation from the first game! It submerges after a few shots or if you get too close. Cheers to ZeroTheEro for this discovery!

The mini-boss is still present and functional, and there's a newly-added second staircase to the boardwalk afterward, though walking up it trap you as the screen refuses to scroll up; a possible entrance to the 8:2? To proceed normally you have to crawl beneath the staircase, allowing you to reach the boss arena, which auto-scrolls but has no boss or moving platform.

8:2

This is a strange, totally original level which takes place on rooftops and balconies. There are several gaps which you can’t jump over without dying, and there’s one bit where you need to use two lives just to reach the other side! There's a number of unused graphics that look appropriate for this area, and they give a glimpse of how the level might have played out.

This would exit you at the stage boss. The entrance was likely either the newly-added second staircase or the ruined balcony after the mid-stage boss; it has collision detection that wasn't there in the original Metal Slug.

8:4

This is the sewer area, just before the staircase. Sound normal? The catch is that the vertical scrolling isn’t working, and you’re trapped; teleporting the player beyond it just presents another dead-end staircase. There’s also some downright dodgy tile placement, such at the pipe with two middles and that bit at the edge there.
Its exit is the same as last time.

9:1

It’s the fourth mission from Metal Slug 1, but it claims to be the third! The third mission isn’t excluded, it’s in A:1.
As usual, you’re given access to a Slug Flyer and Slug Copter, but this level seems rather buggered up. For starters, there’s an infinite number of soldiers storming you from the right side of the screen; second, the destructible cliffs are missing, making the background have large chunks missing. And aside from the vehicles and soldiers, the level is empty.

Metal Slug X / Metal Slug 3

9:2

That area from MS3, the second part of the bottom path in Mission 4. (4:5 2-2) It appears that all they did was simply reverse the background, resulting in the "MODEN" barrels becoming N2COM" barrels.

This would exit you at the wine bar, rather sensibly.

9:4

This is another level that isn’t used anywhere else; it’s a cliff, but with openings in the cliff to show more cliffs.

This would exit you at the start of the mountain range.

Metal Slug X / Metal Slug 3

9:6

Another part of the bottom path from MS3. (4:5 2-2) The palette was darkened and there’s additional detail on the ceiling.

This would exit you before the stage boss.

Metal Slug X / Metal Slug 3

9:8

This appears to be the cave from Metal Slug 3 where you get the Slug Mariner. (3:5 2-1) They changed the palette, reversed the background, and added a few pools of water.

This would exit you at one of the trench entrances before the stage boss.

A:1

Mission 3 from Metal Slug, though the game says it’s the fourth mission! This gives you access to a Slug Copter and Slug Flyer (as usual), and appears fairly normal, other than the typical lack of obstacles and endless waves of soldiers coming from the right side of the screen.
Strangely, the 'cave' on the right wall of the ascent has a black splotch drawn over it that wasn't there in the original game's graphics; a possible entrance to A:2? There are some graphical quirks in the latter half, such as a gap in the background tiles of the enemy camp, and the sky turning solid colours after the destructible building collapses.

Although barely visible, the backdrop of the final section in the ruins features an altered backdrop from the original game! In Metal Slug the beams of light shimmer; in Metal Slug X there are constant explosions in the distance.

A:2

A small snowy area, it looks like it would be part of Mission 2 in Metal Slug 3.
If the exit functioned, it would return you to the start of MS1's third mission.

A:4

The first part of Mission 6, but no obstacles and only one platform.

This would exit you at the sandbags at the enemy encampment.

A:6

A repeat of A:2, exit location and all.

Metal Slug X / Metal Slug 3

A:8

Aha, a part of Mission 3 from MS3! (3:3 1-3) As you can see, there’s a prisoner there, and off the edge of the screen are some unobtainable weapons (they drop off the bottom of the screen), and once you scroll back, a Girida-O and a parachuting platform appear. The prisoner gives you an Armour Piercer.
It was also given serious remodeling in MS3. The palette was darkened, the platforms were redrawn, and those teeny tiny boxes were changed.
It has the same exit location as A:4

B:1

It’s Mission 5 from MS1! You start with a prisoner in front of you, who gives you an Armour Piercer, and after that, some weapons appear. In the order they appear:Another Armour Piercer, Shotgun, Rocket Launcher, Flame Shot, Laser Gun, and Enemy Chaser. Other than another tank on a parachuting platform, this level is fairly normal.

This stage has absolutely no alternate areas; it's the main stage and that's it!

C:1

Final Mission! There’s yet another Armour Piercer wielding prisoner here, and another tank, and other than falling through the ground to reach the boat that is missing it’s turret gun, this level is fairly normal.

Metal Slug X / Metal Slug 3

C:2

This is that part from Metal Slug 3 that involve going through that pipe and riding that ostrich. (3:7 3-1) Looks like all they did was change the palette.

This would exit you after the boat ride, at the stretch before the final boss.