Mr. Pump is a character who appears in the book Going Postal. He is a golem who is assigned to Moist von Lipwig as an assistant/parole officer to prevent his escape from the postal service. He proves ideal for this task, as golems never stop until their task is completed - no matter where Moist might run, Mr. Pump will chase him 24 hours a day for the remainder of Moist's life until he is caught - even walking across the ocean floor to get to him. Moist, who is not stupid, soon realizes this and gives up on his ideas of escape (at least, until he could find out how to shatter a golem with a sledgehammer.)
Like all "younger" Golems (under 3000 or so), he has no actual name; he is designated by his function. Mr Pump previously worked in a hole in the dark for two hundred and forty years pumping water because his creators had never told him when to stop. His description then was "Pump 19". He was eventually discovered by other golems, and brought back to surface. He is also a member of the Golem Trust and is currently working to buy his freedom through inhibiting the freedom of others as a parole officer.
To this end, he is still powered by inserting instructions into his skull. He can apparently track anyone's whereabouts through "Karmic Signature": Moist has several years' worth of fairly smelly Karma and has signed a contract proffered by Vetinari under his own name.
At the end of Going Postal, Mr. Pump is reassigned to apprehend a Randolph Stippler, whom Vetinari also wanted to impress into government service. For all intents and purposes Moist is now as free as could be possible. Albert Spangler would have taken this opportunity to escape and carry on his old life of trickery and deceit, but Moist von Lipwig has different motivations.
Just a hint of Arnie here, as the unstoppable Terminator...