It simply means that since they couldn't afford to do really elaborate prosthetic makeup and costuming for all the pig slaves in that episode, that they made one up with special care and featured him in close-ups. (A hero can also refer to a more detailed and functional prop, as well as a creature.) So actually, Hero Pig doesn't refer to a specific character. It simply means that whenever they had a close-up of a pig slave, they showed a close-up of Paul Casey, though various extras played other pig slaves in the background.
Paul Kasey and Ruari Mears regularly play these prominently featured monsters. Casey, especially, has played pretty much every creature on new Doctor Who and Torchwood. Among other things, he played the blowfish man and Janet the Weevil in the latter show and the Hoix on both shows.
Ruari played a Futurekind in "Utopia", one of the Matron's heavies in the episode "Partners in Crime", a monk in "Tooth and Claw" as well as the earlier stages of the Martha clone in "The Sontaran Stratagem".