Most of the mainline series has been ported to Android by or through the ACA NEOGEO series: Metal Slug 1 : These are widely available as standalone premium apps. Metal Slug 4
Because SNK never officially released a single Android application containing all six numbered titles in one package, the usually refers to a custom-built emulator frontend. These APKs allow players to access the ROMs (Read-Only Memory files) of the original arcade boards directly on their Android devices.
The most reliable and high-quality way to experience the series is through official ports. SNK has partnered with Hamster Corporation to release the series, which offers "arcade-perfect" ports for modern smartphones.
This offers a massive advantage: portability. You can carry the entire history of the Peregrine Falcon (PF) Squad in your pocket, complete with the ability to save states, map custom controls, and even apply graphical filters to smooth out the pixels on high-resolution screens.