Source: Wikipedia, "Metal Slug X", available under the CC-BY-SA License.
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.