A bizarre collage from the Metal Slug X art gallery, chucking a whole pile of sprites and backdrops around the place - including a bunch from Metal Slug 3! X's PlayStation port was released ten months after 3 came out in arcades, but it's still an oddity.
There's the Drill Slug, the Rebel Armor, Elephant Slug, Ostrich Slug, Slug Copter, and the giant falling soldier from the first Metal Slug for good measure.
An unfinished poster only seen in the art gallery. The baby and alien abduction suggest it's X, but then there's an elephant in centre frame. You're not in this game, buddy!
Concept art is sourced from the Metal Slug X PlayStation Art Gallery, Neo Geo Freak July 1999 (thanks to Divison 六 for the scans!), the Metal Slug 6 Art Gallery, the Metal Slug 10th Anniversary website and the Metal Slug Attack Illustration Gallery.