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!
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.
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 they are copies of it rotated in around 32 directions.
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
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.
The Metal Slug's vulcan cannon detached and sparking on the ground. One can assume this would happen after the tank is destroyed or during a Slug Charge attack.
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.
There was an entirely unused tank hidden inside the game! It’s style is very reminiscent of In The Hunt, a game made by Irem that apparently was made by the Metal Slug staff before moving over to Nazca. Ivo informed me that this tank was the Regular Army’s MBT The Sand Simon, and then flip-flopped and said it was probably just an earlier version of the Di-kokka, the rounded fat tanks. If it were a Regular Army vehicle he believes the tank should be white, like so.
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.
Bullet cases found near the turret sprites. Presumably a visual feature that never got implemented.
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.
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!
This cartoony grenade-juggling monkey lurks among the HUD and the text, and appears there in just about every instalment. It's used on the loading screens of the original console ports of the first two games.