The Manpigs, much like Grunts and Brutes, are deformed, pig-human hybrids. They patrol certain areas of the facility, however if the lantern is shone on them, they will pursue the player relentlessly. The lantern can also alert Mandus, because it will start to flicker if a Manpig is nearby. The same effect applies to virtually all electrical lamps. This is because Compound X, which was being used to facilitate bonding between different cells (in this case, human and pig); is highly light reactive.
The Manpigs obey the machine only, as shown when it ordered them to slaughter the population of London or directly when it ordered them to kill Mandus. They also seem to have the intelligence level of a small child as evidenced by Oswald seeing them playing with toys and even calling them his children on occasion.
It is possible that the Manpigs are direct descendants of the Gatherers, so to speak. This is because, in one document, Mandus describes a body delivered to him in a crate that matches the description of a gatherer. Furthermore, he mentions how he can "smell the orb upon him", and that this is the "work of his great uncle Alexander".