A massively multiplayer online game called Metal Slug Zero was being developed by WIZ Hands back in 2009. The project was going to be a top-down shooter with online capabilities. It was later cancelled due to lack of progress by WIZ and, according to the publisher Dragonfly, did not meet industry standards.
The only character transformation that does not appear in the regular arcade mode in any of the titles is the Super Devil, which makes the player extra powerful with a special weapon. This form can only be obtained in the Combat School mode of Metal Slug, Metal Slug 2 and Metal Slug X. Concept art of this form resembles a green demon, although all it does visually is change the character's color, making them resemble Metal Slug 3's clone enemies.
Eri's full name, Eri Kasamoto, is based on the name of the actress Rie Tomosaka. Eri's name is simply an anagram.
Marco Rossi and Fiolina Germi, in addition to her teddy bear Peppino, have their names derived from the 1976 anime series "3000 Leagues in Search of Mother".