"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.