A mouser is a cat that is kept for the purpose of catching mice.In England,ther has been a mouser at the Prime Minister's residence for centuries.The most popular of these cats was a stray named Humphrey who,in 1989,was the first cat to receive the official title"Chief Mouser to the Cabinet Office."His care cost100pounds and was paid from the department's budget.lt was said to be far cheaper than the fees previously paid to exterminators.humphrey,who was beloved in the UK, retired in 1997. The current mouser,named sybil,took office in september of 2007