What better way to start off a series than by having a heap of unused crap lurking inside? Findings are sorted by whatever graphic comes first in the tile viewer.
Props goes to Ivo, V-King, tango_311, stoffhime1 and Magma Dragoon for finding a lot of this stuff.
Source: 6912 (8x8 view)
Not even Metal Slug escaped from William S. Sessons' campaign of drug awareness! ... except it goes entirely unused and isn't seen in-game at all. Whoops!
Source: 891 (8x8 view)
This cute little Slug turret is found at the end of the menu alphabet, and actually dates to a prototype of the game where you only played as the tank - see early footage for details!
Source: 896 (8x8 view)
And I can't find the right palette for this (it's meant to fade from yellow to red), but this is a health bar from the aforementioned early revision.
The very second sprites stored in the graphic data (right after the Rebels' landmine explosion); what seems to be an early hitspark for when the Metal Slug took damage.
S, G, H and M icons. Placeholders or early HUD icons for the weapons, presumably.
A large, rotating Nazca Corporation logo (you'll have to take my word for it on the rotating part! check out Magma MK-II's sprite sheet for the full thing). The developers looked like they'd planned to make it clear as mud who was responsible for the game, but it goes unused.
A jingle and soundbyte for this alleged screen can be heard in the sound test at 2C; it was also included as the first track of the Metal Slug Complete Box soundtrack - thanks to Stefano for this factoid!
Animations for the dogs from Mission 5 walking and sniffing, found by Ivo. You only ever see the dogs running in-game, though the early footage appears to show one of these animations in action.
A teeny tiny flopping fish, found by Ivo. Might be used somewhere for all I know.
Ivo found these barrels in the vicinity of Mission 2's graphics. Are they used anywhere?
Division 六 found these unused frames from Morden's sprites in the 2-player ending; these would make a better transition from his idle stance to him seeing the paper plane, but instead he just cuts between two different poses.
An elk. Presumably for use in Mission 3 or the ending movie scenes.
It appears in the Metal Slug Official Guide Book character chart, representing the decimated wildlife that have it out for Morden.
A very peculiar sprite of an enemy soldier with a distinct uniform. No one seems to have found a fitting palette for it that colours the mouth and eye (visor?) correctly. This is the only animation it has, though there are copies of it rotated in around 10 directions. Ivo suggests it might be the character depicted in this concept art. It's possible!
Divison 六 suggests it may have been the Metal Slug pilot from before Marco and Tarma were introduced; the sprite fits perfectly into the chassis during the Slug's "damaged" animation, and the multiple rotations fit with how many directions the tank can face. It's possible too!
This stupid kid has appeared in all of the games' data, but not actually in gameplay. tango_311 provided the real palette.
These three ladies only have one frame of animation each, and it goes without saying they're unused or else they wouldn't be on this page. It's also nigh-impossible to find decent palettes for them. tango_311 got these superior palettes for them, though the last two have rather punk colourations for rural villagers
The Metal Slug's tiles are arranged strangely so this animation is a bit splotchy, but it appears to be firing its cannon while crouched; using the bomb button while ducking will just throw a grenade. This appears to be a leftover from the early development when the Metal Slug was the only playable character; early footage suggests crouching and using the cannon would have fired a different projectile!
Source: 11339, 20057
The game is already dripping in rotated sprites... but I can't think of a purpose for these tilted splashes. There's a watery slope in Mission 1 but it just produces regular hitsparks, not splashes.
What appears to be a cargo lift, probably for the Metal Slug or enemy tanks. Found by Ivo. Not sure what it woudld be supported on - a helicopter? A crane?
The Metal Slug's vulcan cannon detached and sparking on the ground. One can assume this would happen after the tank is destroyed, but it explodes in mid-air no matter what.
The Metal Slug has ludicrous amounts of rotations - more than are actually feasible in-game! This is but one example, there's way more than I could possibly cover. Thanks to Ivo for this trivia.
Located right before the graphics for the Melty Honey. The driver actually loads these shells during the reload animation (), but fires the far larger homing missiles; likely replaced for the sake of visual clarity. Cheers to Divison 六 for this find.
An entirely unused tank is hidden inside the game! Its style is very reminiscent of In The Hunt and Gunforce 2, games made by Irem that apparently was made by the Metal Slug staff before moving over to Nazca.
Named the "KT-21" in concept art, Ivo believes it was an early version of the Di-Kokka (the rounded fat tanks) - its animations hit all the same beats, from its recoil-heavy fire, its wobbly movemeent, and its hatch opening when it's destroyed. The Di-Kokka even has an unused tank shell very similar to the one above, if you read on!
An unused shell found between the tiles for the Girida-O and the Iron Iso; it's possible they used this missile instead of cannonballs. I believe these remain unused in Metal Slug 2 and X.
The Iron Iso turning around. It's possible it would play this in-game, but the camera never gives you the chance to get behind it - not until Metal Slug 4, at least. Cheers to Division 六.
Located after the Iron Iso's destroyed animations are these frames of its turret separated from its chassis.
Early footage suggests when destroyed its turret would have popped off, similar to the Girida-O.
Found after the Iron Iso's graphics are these trailed projectiles... and immediately after them, the orbs the Iron Iso uses in-game for its projectiles.
A sandbag with a wider, shorter profile. Not unused, just stupefyingly obscure - it's found inside the shack at the beginning of Mission 4! It's fully functional and only became visible to me in the first test stage. Thanks to Ivo for pointing the object out to me, and Division 六 for its appearance in Mission 4!
This animation is used but rarely seen - in 2-player mode you can backtrack slightly with the Metal Slug and see this animation when the rock trap lands on it. Cheers to ZeroTheEro for this factoid!
The turret outposts in Mission 4 have four unused variants with varying cannon ports and amounts of foliage. The top-left design would later be edited for use in the second mission of Metal Slug 4. Cheers to Division 六 for the find!
Source: 42942, 47470
What we have here is a Rebel soldier using a turret. Although they do use missile turrets in Mission 5, they don't use the same turrets you use in Mission 6. Has all the necessary animation, too!
Curiously, Marco and Tarma's turret animations immediately follow these sprites. All of their sprites are otherwise found at the very end of the sprite data in one organised lump.
A parachute Ivo suggested belonged to a Slug. If true, there really must haven been more emphasis on the 'chutes early in production, what with all those animations for Marco and Tarma while parachuting. There's more frames, of course, but he couldn't be bothering ripping them. Wise man.
A larger water splash for the Middleson boat seen in Mission 6. Cheers to Division 六.
An armoured car with a tank's entrance hatch and sandbags, falling. Judging from the sandbags, I'm assuming these were for the stronghold in Mission 3, but that's very little to judge off.
Ivo also made a second version with a palette that actually existed in the game, but it looks like ass in my view. Diss!
Ivo found this cannon shell near the Di-Kokka's graphics; early footage shows them firing these instead of their trademark fireballs. The fireballs are among the last sprites in the game's data, just before Marco and Tarma.
An incredibly elaborate explosion or dust cloud. Used in the tank-only prototypes for the Metal Slug's cannon explosions.
A dustcloud, probably kicked up by the Metal Slug. As far as I'm aware this visual effect doesn't exist in the first game (I thought I'd seen it...!), and later games use a less elaborate graphic.
Seemingly an early animation for the shotgun blast.
Found among the Metal Slug's tilted cannon blast animations. Intended for the Metal Slug's rolling projectile from the early prototypes, perhaps?
Found near the graphics for the Hairbuster Riberts (Mission 2 boss); an exhaust intended for the missiles, but goes unused. Cheers to Divison 六 for this find.
Source: 77211, 77584, 77955
Source: 77325, 77345, 77840, 79514
Ivo found animations of the characters shooting and throwing things in many directions while in the air, but I got these and animated them. It appears that the characters would've been more flexible, turning from every angle, instead of just turning their whole body around to whatever direction.
What's interesting is that just about all of these animations are still in the game! When your character parachutes in (in Missions 1, 2 and 6), release them from the parachute, aim left, and fire. Throwing grenades also works, and the Combat School in the console ports allows the machine gun wielding animations to be used thanks to you keeping your weapon after finishing a level. Thanks to V-King for most of this info.
Source: 77612, 77631, 77680, 78390, 78444
These turning animations, however, cannot be seen in-game.
Some of these animations were modified and repurposed for the unused helper character in Metal Slug 5... who also went unused, you might have gathered. They finally appeared somewhat as intended as the Special Marco unit in Metal Slug Attack.
This cartoony grenade-juggling monkey lurks among the HUD and the text, and appears there in just about every instalment (at least, I'm sure it used to - have newer ROM dumps excised it?). It's used on the loading screens of the original console ports of the first two games.