The creatures are technically modified pedestrians, wearing shorts, programmed to spawn in San Andreas swamps. Upon emerging from the Swamps, they tend to attack the player.
The myth is part of the famous Mythemix modification, which adds numerous false myths into the game. The Swampmen, despite being false, are investigated by many users. Some videos have also hoaxed their sightings using the mod, only to worsen the already false status of the myth.
Swampmen in the mod could be based on the Swamp Thing from DC comics, a creature that also inhabits the Swamp, but unlike the pedestrians in the mod, Swamp Thing is monster-like in appearance.