Zombarian

The Zombarians are a humanoid alien species which conceal their faces in masks and wear what appears to be respiratory apparatus; however they have been shown to be perfectly capable of surviving in Earth atmosphere unmasked for extended periods of time. They are known to form an interstellar police force which has collaborated with the MiB on several occasions; however some members of this species are outlaw bounty hunters. They apparently use ammonia as currency, or at least consider it valuable enough to be used as payment for bounty hunter services.