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You may be looking for the Arcateenian.

Mary was a prostitute in Cardiff in 1812. Mary insulted a soldier, saying that she wasn't his "hound." She attacked him and ran away. Mary found an Arcateenian in the woods. The alien took over Mary's body and used it to leech the life force off of others for the next two hundred years before being killed by Torchwood Three operative Jack Harkness. (TV: Greeks Bearing Gifts)