This is a list of some sprites that were not used.
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
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.
The real final boss of Metal Slug 5, scrapped due to the game being rushed.
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 manipulates the Ptolemaic army from the shadows. He alone annihilated the 'Red Goblin' secret society.
A undefined prototype that may be a Slug or an enemy weapon.
It resembles a Messerschmitt Me 163 Komet, a German WWII fighter.
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 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.