The Neo Geo was unique among arcade systems. Unlike a dedicated game board (e.g., Street Fighter II on CPS-1), the Neo Geo MVS (Multi Video System) was a cartridge-based motherboard. The BIOS lived on the motherboard, not on each game cartridge. When you load any Neo Geo ROM— Metal Slug , King of Fighters , Samurai Shodown —the emulator must first load the BIOS to initialize the system.
MAME uses a merged BIOS system where neogeo.zip contains far more files (including memory card dumps, sound CPU ROMs, etc.). FBNeo is leaner. If you drop a MAME neogeo.zip into FBNeo, it may work—or it may fail due to extra/unexpected files. For guaranteed results, source a set labeled . fbneo neogeo.zip