A revised version of Metal Slug 2, titled Metal Slug X, was released on March 1999 for the Neo Geo MVS. It fixed problems with slowdown present in Metal Slug 2, and increased the difficulty. Metal Slug X also introduced a few new elements to the Metal Slug 2 game system. New weapons and items were added, such as the "Iron Lizard" and the "Drop Shot". The enemy placement and bosses were re-arranged as well.
Differences from Metal Slug 2
* Much of the music has been slightly remixed or altered.
* Many stages had the time of day changed. Instead of being simply at day or at night, the stages can take place at dusk, twilight, or sunset.
* It is now possible for the player character to become fat by collecting food items in every stage after Mission 2, while it was previously only possible in Mission 4.
* There is less slowdown than in Metal Slug 2.
* All levels contain an increased enemy count and changed enemy placement as well as enemy characters themselves being changed, including altered boss placement.
* Vehicle types and locations are usually changed.
* There are generally more power ups, prisoners, and items (particularly food), which are often hidden and must be shot to obtain.
* Many environmental elements have altered reactions to being shot, such as exploding with unexpected results or randomly spewing out items or enemies.
* Heavier damage versions are available for the heavy machine gun, flameshot, shotgun, laser rifle, and rocket launcher. When compared with their normal counterparts, each big/super weapon deals more damage, has a more "fierce" appearance, and has a wider (or longer) area of impact. If the player happens to be fat while using these heavier weapons, a new appearance takes on and the player does even more damage.
* Several new weapons were added to the game: Stones, Iron Lizard, Enemy Chaser, Super Grenade, and Drop Shot. A new Metal Slug, the Golden Metal Slug, was added to Mission 3.
* The original art for Metal Slug is shown at the end of the game while the credits are rolling, instead of a black screen used in Metal Slug 2.
Source: Wikipedia, "Metal Slug X", available under the CC-BY-SA License.