M.U.G.E.N. (Or Mugen) is a sandbox fighting game engine created by Elecbyte in 2000. It allows people to create a custom fighting game with the characters and stages they want, and they're also able to create their own characters.
There's some examples of many Metal Slug Characters and Stages.
Final Mission (Metal Slug X)
Morden's Army (Original)
Death Force (Dragon Nosuke edit)
Slug Wanzer (Slug Gunner edit)
Mars People (SVC Chaos)
Marco Rossi (Neo-Geo Battle Coliseum)
Trevor Spacey (Original/edited)