A pig-rat was a kind of non-sentient rat which lived on Gallifrey. They could eat through Time Lord flesh, at least according to a threat Janartis once levelled against Romana II. Indeed, Romana's home of Heartshaven was once overrun by them.
The Free Time Activists bred the Dogma Virus in pig-rats. The virus remained dormant until regeneration, at which point the Time Lord would be infected and would chant "Free Time". Romana and K9 realised that the infected Time Lords were building a structure resembling a giant pig-rat nest in the catacombs. Braxiatel presented the pig-rats Gillestes used to develop the antidote, as they contained the cure; however, as the virus remained dormant until regeneration, the only cure would be to remove their ability to regenerate. (AUDIO: Panacea)