"Cash Cow" was a name given to a large creature used for illegal meat production near Cardiff on Earth in 2008. Its actual name was unknown. It was believed to have been taken from the sea by members of Harries & Harries when it was small, but due to a mutation which caused it to constantly grow, it soon reached an immense size and nearly filled the warehouse it was kept in. This mutation allowed the producers to harvest meat from it while it was still alive, as it would just grow back. Harries & Harries believed it was a non-sentient Terran creature from the depths of the ocean; however, Jack Harkness believed that it was not from Mutter's Spiral at all.
When they first learned of it, most of the team felt they should destroy the creature, or as Tosh put it, "Put it out of its misery". They also could not see any use for it in defence of its future. Jack, however, said they should save it, as he felt it wasn't a threat and was just lost and being abused; that it was their duty to protect and help it. Jack said once they rescued it, they would nurse it back to health, theorising the mutations would stop if it was no longer hurt. Once healed, he planned to send it back through the Rift.