Unused Graphics

Metal Slug Advance has unused sprites, but they're really nothing new. JUST LIKE THE GAMEPLAY AM I RITE
And because the tiles in this game are so weirdly laid out, don't be surprised if the sprites look chopped and unfinished. I don't know if it's intentional, but it bugs me so.
Palettes can be found at offset 79CCC.

The characters once reloaded, apparently. The last frame of this animation appears after Walter shoots forward in the game.

Tyra and Walter apparently once had second crouching animations, with their guns at the ready. The arcade games have multiple crouching stances, but Metal Slug Advance limits the characters to just one.
Okay so about one or two frames are used after firing the shotgun or super grenade but I'm still saying they're unused just because.

Unused looking downwards?

The missing crouch + slice that was seen in one of the videos! Yep.

Using Marco and Tarma's kick attack.

And Tyra, lynching Eri's karate kicks.

Unused looking up animation, I guess.

Some unused looking up + turning frames.

And also in the arcade games, your character takes a few small steps before running, and those frames are here, but aren't used.

And an unused waiting animation, of Walter taking a slug from his canteen.

And here he is being a lazy git.

And using the radio. There doesn't appear to be a version of this for his pistol.

And a crouch + slice that wasn't seen in the videos! Yep again.

Now I didn't expect this, ever: Walter using an unused Metal Slug 1 animation! And this is unused too. How ironic?

And an unused waiting animation for Tyra, where she checks her hair or something. This gets a plus point for not being ripped off an older sprite, but gets a massive minus for being a crappy animation.

This animation later found in Metal Slug Defense. Thanks to @neokyiser for the heads-up!

Tyra has all of her frames for jumping with the pistol, but the game only uses one. Crazy!

source: 52ACE8, 52E93C

An arm for throwing grenades from a vehicle, stored with Walter and Tyra's Slug Flyer graphics. Grenades just appear from the void without impetus.

[there's also a second version of the duo's walking animation, which is exactly the same except for some pixels changed. I ain't making a new image for a couple of pixels, yo.]

source: 588922

Dah dah dahhh!! The Slug Mariner! More evidence that there was once an underwater level in the game, before they scrapped it and so on. It even has all the necessary animation for being used in the game, though there aren't any corresponding graphics for Walter and Tyra to pilot it.

source: 7B7BA4

Another leftover of the aquatic level! This time, it's the squid from MS5. It also has all the necessary animation, including it exploding in it's gory fashion.

source: 7C08BC

And another leftover, a mine. Astonishing. It's worth noting that the squid and the mine are the second-to-last graphics stored in the ROM; the poison gas in Mission 3 is the absolute last.

And the last of the underwater elements, a boulder with no debris.

Hyakutaro. It would've been handy to have that guy in the game, what with there being absolutely no allies at all, unless those dipsticks in tunics who have to be rescued from cages and ropes and invisible hotspots count as allies.

This hand was used for the menus on the console versions of the other games, so I presume it would've been used in the menus of this too.

source: 8B5F0

An unused HUD element for the Black Hound's missing laser. I'm still proclaiming the lack of that to be the game's downfall, by the way.

And the In and out icons. No big deal about them being left out, since they're barely important. When your vehicle begins to shake, flash, and emit a siren, I think it's pretty obvious that you need to get your ass outta there.
And I didn't change the colours this time. Rejoice?

[there's also a Japanese soldier running without his sword raised, but I think we get the point, don't we?]

Destructible wire fences. Seems a good fit for Mission 5 or the dungeon. Cheers to Ernesto Aguirre for the find.

Destructible target practise signs. If we were to see the PF cadets in action, these would fit right in. Cheers to Ernesto Aguirre for the find.

A PF cadet in shackles! An all-new animation not repurposed from unused soldier animations, to boot! Few places it could fit in, though.

Rebel cannons on wooden constructs. No doubt intended for Mission 2. Cheers to Ernesto Aguirre for the find.

Various bits of cavern and temple decor that seems right at home with Mission 3; it's possible they appear in-game as background tiles, but these are expressedly sprites. The stone face is depicted on the Pandora card. Cheers to Ernesto Aguirre for the find.