This must have been a really early design of the Slug Tank. The crew is unprotected, no hydraulics that allows the tanks to jump are visible, and there are no armor platings that shield the threads.

From Metal Slug 1.

Arts for mission 1 of Metal Slug 1. If you look at the Slug Tank in the pictures, you will notice that its pair of Vulcans are missing, as well as strange barrel sticking out of the rear and missing piece of hardware on top of the turret.

The threads also look weirder. Also, the two things that stick out from the side of the thread shield are missing, too.

From Metal Slug 1.

Early, early, early, and applied concepts of the Slug Tank. The early versions look much more cartoonish since most of the parts are round and seperated. The applied concept is more rigid and tighter looking.

From Metal Slug 1.

The above pictures are in relation to the never-used SV-002. It's more sleek, and has a machine gun and a missile launcher instead of a vulcan. Its cannon's caliber is smaller, and the chassis profile is much lower. The headlights have also moved further to the rear of the top of the turret. The mineplow is also higher off the ground, and the all-bearing antenna has been changed from being cylindrical shape to a shape that of a cutter.

From Metal Slug 1.

The Middle-Son class submarines. The older variant, Middle-Son 1986, has been used in the final mission of Metal Slug 1 after you fall down the bridge when Morden blows it up.

The newer one, Middle-Son 1999, is used in the first mission of Metal Slug 1 as a derelict submarine.

From Metal Slug 1.

To the left is the old design of Type-2 Di Cokka. In the middle is a very funky design of it that got scrapped. To the right is the one that has been used for the final game.

The picture to the right shows how close the tank looked like M3 Lee battle tank used by the Americans during World War II.

From Metal Slug 1.

Early, early, and applied concepts of the Type-2B Melty Honey.

The first picture shows the Melty Honey as a battle tank. The ones in the second and third makes it resemble a fire-support vehicle, with its open top crew compartment. The difference between the second and third designs is that the third has the spikes on its front armor, while the previous one does not.

From Metal Slug 1.

To the left is the old design of the Type-3 Bull Chan. To the right is the one that has been used for the final game.

But more importantly, the first picture shows how the designers were planning to add more sophisticated projectile/armor characteristics into the game, seeing how the shell is being bounced off of the top armor of that vehicle.

From Metal Slug 1.

Early, early, and applied concepts of the Type-4 Girida-O battle tank.

The picture in the middle makes the tank resemble the M4A3 Sherman used by the Allies during World War II.

From Metal Slug 1.

Early and applied concepts of the Type-5 Iron Iso assault tank.

The first picture makes the tank resemble the German SdKfz173 Jadgpanther, although the latter resembles the PzKw VI Tiger.

From Metal Slug 1.

Anti-aircraft gun. There was suppose to be another AFV for the Rebellion designated as "Type-6" which serves as an anti-aircraft vehicle. But it seems that it was scrapped due to the fact that there were no controllable aircraft for the players in Metal Slug 1.

From Metal Slug 1.

Another mobile ack-ack gun that never made it into the game.

From Metal Slug 1.

Early, early, early, applied, and applied concepts of the Tani-Oh, the level 3 boss from Metal Slug 1.

The first picture shows a blocky tank with three turrets and a lot of and barrels. The second image shows a huge tank with just a huge turret with a huge gun. The third shows a huge turret mounted on its chassis that has even more smaller turrets with cannons mounted on it. It also has guns located on the side of its chassis. The final two, however, shows a completely different picture with the majority of its smaller cannons replaced by just two monstrous guns with two minelayer/Vulcan pods on each end.

From Metal Slug 1.

Early and applied concepts of the Shoe & Karn, the level 4 boss of Metal Slug 1.

Note the difference in armaments between the old and new designs. The old one carried an anti-aircraft cannon on the back instead of a cruise missile. The new design also has a longer barrel for the gun, as well as twin auxiliary guns below it. Mineplow has been added to the new design, and the new turret now resembles that of the Girida-O's.

From Metal Slug 1.

Both are named Land Seek, although the early concept, which is suppose to resemble American World War II GPV, is replaced by a simple transport truck that resembles German Opel truck of World War II.

From Metal Slug 1.

A destroyed World War II German PzKw IV Ausf. H Panzer tank, which is definitely not in the game.

From Metal Slug 1.

Well, would you look at that. Mazinger Z with an ugly face attached to it.

Not used in the game, but it says "final weapon" at the top right of the picture. I would love to cream that thing and mop the floor with it using a heavy machine gun.

From Metal Slug 1.

Although you can spot MV280B in mission 4 of Metal Slug 2 and X, you will not see any MV280A unless you manage to shoot off the MV280B's rocket launcher turret with explosives. And even that's rare enough.

When its rocket launchers are destroyed, the jeep will just try to ram you instead.

Early, applied, applied, and applied concepts of the R-Shobu. The first and second pictures make the damned thing look like a child's toy, while the fourth makes it look like a joke, and the third and fifth making it much more respectable than others.

Notice how the aircraft looks considerably different by comparing the fourth picture to the others.

Early, applied, applied, and applied concepts of the Flying Tara. Strangely, the second/third and fourth concepts do not go along with each other, even though they are suppose to be the final concept of the unit in the game. The second/third picture shows a fighter-bomber, while the fourth shows a dive-bomber.

Early and applied concepts of the Hairbuster Riberts, level 2 boss from Metal Slug 1 and level 5 mini-boss from Metal Slug 3.

Not much has changed in design, except that the two engines located on its wings were replaced with a single engine that is placed on top of the aircraft.

From Metal Slug 1.

I have...

...No clue...

...What these...

...Are suppose to be.

Gundam spin-offs, maybe?

But I do know that there was a seperate project for developing a spin-off of Metal Slug 1 that was going to include these robots. However, the project was cancelled due to lack of interest from the staffs due to the fact that Metal Slug 1 was suppose to be World War II oriented, not sci-fi.

From Metal Slug 1.

A Bicycle Slug.

"Slaughtering the enemy while excersizing for fitness!"

Fine, I admit that the above line is not funny.

But you know what, I'd like to see more pictures of Fio like that.

From Metal Slug 3.

Car Slugs. They look quite different from the one in Metal Slug 5.

This is from Metal Slug 3, by the way.

A Turtle Slug. Maybe it was suppose to be used instead of giving the players Slug Mariner every time they went into the water.

From Metal Slug 3.