This page is a list of sprites found in the game files of the series that were not used by SNK for various reasons.
POWs that were supposed to support the player with powerful guns but always stay behind you. The one on the left is Achilles, and the fat one is his comrade, Tabomba. Both unused characters from Metal Slug 5.
It is an obtainable unit in Metal Slug Defense and Metal Slug Attack, complete with animations. After the events of Metal Slug 5, the remnants of the Ptolemaic Army also created an offensive variant called Emerald Turtle. It is often used as a form of excavator too.
An unused rideable vehicle in Metal Slug 1, but was scrapped for currently unknown reasons. It is speculated to be a prototype of the SV-001. Fans even speculate that Rebel spies stole data on the Sand Simon and developed the Di-Cokka.
Even more evidence of this exists as one of the early designs for the Di-Cokka shows it with two cannons, just like the Sand Simon. It has never made a comeback.
One of the final bosses of Metal Slug 5, scrapped due to the game being rushed. He should have appeared before the summoning of the Evil Spirit Incarnate, once the players arrives on top of the Osaka Tower.
This unit is eventually obtainable as part of a special event in Metal Slug Attack. Like the Stone Turtle before him, he is now a useable in an actual Metal Slug game, complete with animations and a proper name as well.
His lore mentions that he's a mysterious priest who leads and manipulates the Ptolemaic Army from the shadows. He alone annihilated the "Red Goblin" secret society.
An undefined prototype that may be a Slug as well as an enemy vehicle. It is however speculated to be one of the fighter jets used by the Regular Army.
It resembles a Messerschmitt Me 163 Komet, a German WWII jet fighter, but with a more modern look and weaponry.
A unidentified object, possibly from a large machine or scrap.
An old man planned to be used in one of the Metal Slug X's unused stages. He would scold the player when close to his window and will react when fired upon.
4 secret images.
It appears that the characters were first considered to be 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.