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 Robotnik, 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.
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!). 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 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?
An elk. Presumably for use in Mission 3 or the ending movie scenes.
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!
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
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.
A variant of the knifing animation with the soldier snarling happily. I almost think it's used, but we all know how many times I've been wrong.
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 "Sand Simon" in concept art (that i can't seem to find, welp), 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 short-and-blocky tank and the large-and-immobile tank (yeah, pardon me, I've forgotten all these delightfully obscure names for the vehicles). I believe these remain in Metal Slug 2 and X.
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.
Found by Ivo: a soldier throwing a pipe bomb while running! Enemy soldiers have always been rooted to the ground while throwing grenades throughout the entire series, so this is pretty wild. Ivo says there's no variant with the final 'pineapple' grenades, so it must have been made only for the tank-only prototype.
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 small copyright graphic for Nazca, probably a leftover from an early title screen.
Bullet cases found near the turret sprites. Presumably a visual feature that never got implemented.
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 projectile by the Di-Kokka's graphics; rather than shooting fireballs, it must've fired more traditional shells. (the fireballs are among the last sprites in the game's data, just before Marco and Tarma)
Source: 77211, 77584, 77955
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. The turning animations, however, simply don't exist.. Thanks to V-King for most of this info.
Some of these were reused for Glen Achilles in Metal Slug 5.
Throwing a grenade forward.
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.