Rupert Bleakley is a vampire. Presumably part of the Ankh-Morpork League of Temperance, he works in what would, on Roundworld, be a most unusual place. Presumably he sucks the animals dry quicker than gravity could expel the blood for them.
At a time when Sam Vimes is more than usually annoyed, he tells Fred Colon that he'd even swear in a 'damn gorgon'. Fred's reply is cut off very quickly: "There's always Mr Bleakley, sir, he's getting fed up with working in the kosher butcher's and—"
"But no vampires. Never any vampires. Now let's get a move on, Fred."