Concept Art

"Black Player", versions of the player characters with blank eyes; likely concept art for the evil clones aboard the Mars People mothership. The third page has "Dark" versions, with modified outfits featuring chains, spikes and ridiculous shoulder pads. The clones use their own separate set of graphics in-game, so it's possibly they were to be edited further beyond just removing their pupils.

"Bicycle Slug" (ちゃりスラ): a regular push-bike with a vulcan cannon attached to it.

On top, a gnarly robot armour, though it's unclear if it's for the good guys or bad guys.
Below, the "Stratoslug / ストラトススラッグ" (left) and the "Abarth Slug / アバルトスラッグ" (right), based roughly off the Lancia Stratos Group 4 and Fiat Abarth 131, respectively. We wouldn't get a car slug until the Slug Mobile in Metal Slug 5.

The "Turtle Slug / かめスラ". Maybe it was supposed to be an alternative to the Slug Mariner, similar to the Slug Copter.

Concept art for the Mono Eye and their UFO from Mission 2. It shows what lurks beneath the giant structure - a load of nasty tentacles!

Rootmars is depicted in this concept art with disgusting tubes poking out from every corner, which are missing in the in-game sprite.

A big mother jellyfish. You never encounter these in the wild, they're only seen being chomped by the giant eels, though you'd later fight them in Metal Slug 5.

Concept art is sourced from Neo Geo Freak July 2000 (thanks to Divison 六 for the scans!), Neo Geo Freak August 2000 (ganked from Enomosiki's Metal Slug Database), the now-expired Metal Slug Project Omake Corner, the Metal Slug 6 Art Gallery and the Metal Slug Attack Illustration Gallery. A big thank you to Divison 六 for providing such large clean images!