There's not a spectacular amount of unused content in Metal Slug 2 - a lot of the tiles are merely leftovers from the previous instalment.
A general note: the graphics in Metal Slug 2 are very strangely organised. Player graphics are strewn all over the place and arranged in contradictory manners - sometimes graphics for Tarma and Eri are in the same "full" assembled manner as they are in later games, while at some points they're kept as "headswap" tiles, where only the relevant chunks are stored to be applied to Marco or Fio's graphics. Because of this, some animations are repeated multiple times through the graphic data. I can't really keep track of everything.
Not unused but merely unseen, the destroyed bus in Mission 4 has this rough outline hidden behind a layer of rubble. The text reads "優先低"; roughly "low priority" or "bottom layer".
Right at the start of Mission 6 is a tower with a searchlight on top. These unused graphics suggests the tower was destructible at one point, but it can't be interacted with. Ta to Division 六 for the find!
Ivo found the cat and esreveR ripped it. I've no idea where you'd see the cute little thing, though it is depicted in the concept art, possibly as an item.
Each of the animal Slugs have two different win icons: One of them standing, one of them sitting. The sitting Camel Slug one is used, but not the standing one. Left over in Metal Slug X.
The Eaca-B's sprites were updated for this game, boasting lots of new frames of animation for its aerial rotations, and even applying those luxuries to the propellers and Flying Tara's bomb. Unfortunately, the Rebel planes no longer perform loop-de-loops after the first Metal Slug, so all this work was for naught. Credit to The Huntress for this observation, whose sprite sheet shows all the relevant frames.
Fio using Eri's win animation. Given the use of "headswap" tiles, this was probably made as a base for Eri later, perhaps.
Abul Abbas wringing his sword mournfully. Presumably he would do this before waving his surrender flag.
Abul Abbas stabbing with his sword. This is stored at the end of his walking animation, and it appears to flow smoothly in and out of it. As it is, the only time he's walking is when he's escaping off-screen, so he never interacts with the player characters.
These two animations later saw use in Metal Slug Defense. Thanks to The Spriterminator for the tip.
Divison 六 found these frames suggesting the Ajirabian soldiers could've performed a crouching stab, possibly after somersaulting. According to SoldierRebel they will actually use this animation when the difficulty is set to 8 while somersaulting out of the Keesi, but I've yet to see it myself.
Another Divison 六 find of the Ajirabian soldier seemingly climbing off the camel (it's found in the proximity of other camel-riding animations).
This is apparently the Aeshi Nero/Mission 2 boss's laser thinning out. But, uh, yeah, I can't understand a lick of these tiles. Thanks esreveR?
The Dragon Nosuke, the boss of Mission 3, has eight directions with unique idle and firing animations for its fireball-spitting cannon. Ta to Division 六 for the find!
According to Ivo, this little scarab beetle is unused. Can't remember which games it's in. Also has a flying animation, thanks to Domino for pointing it out.
esreveR pointed out to me that Mission 4's boss, Big Shiee, never has a flame trail on these animations. Yup.
tango_311 found these in MS2 near the graphics of the bikers, presumably to be used as part of their debris, but they aren't used at all.
This isnít really an unused sprite, but you never get to see it properly, so I guess it goes here. Itís the animation the soldiers play when they kick down the doors of the carriages in Mission 3. Found by Ivo, in both MSX and MS2.
Here's an incomplete list of leftovers from the first game.
|1024 (8x8)||Loading screen monkey (used in NGCD version)|
||Winners Don't Use Drugs
||Marco throwing backwards
||Marco's unused crouch throw
||Marco aiming up (Heavy Machine Gun)
||Tarma's unused crouch throw
||Soldier using a hatchet (Mission 1)
||Tarma aiming up and turning (Heavy Machine Gun)
||Soldier using a pipebomb
||Soldier burning (MS1-style)
||Soldier in outhouse
||Soldier using knife, happy expression
||Large graphic of Soldier (Mission 3)
||Soldier in rubber ring
||Mission 1 jungle scenery
||Soldiers humping tank
||Tarma throwing backwards
||Tarma shooting backwards (Heavy Machine Gun)