First introduced in Duke Nukem 3D, the Pig Cop has become one of the most well-known symbols of the Duke Nukem franchise. Not much is actually known about these monsters, other than the L.A.P.D. (or L.A.R.D. as they're called in the games) have somehow mutated into pigs when the aliens invaded Earth.
From a design point of view, it is believed that 3D Realms made them this way as a comic remark of the common notion, from American pop-culture, that the cops are pigs.
Throughout Duke Nukem 3D and its expansions, the Pig Cops are sometimes encountered flying a small plane. In the Plutonium Pak, the Pig Cops were giving a riot tank to play with as well.