A mouser is a cat that is kept for the purpose of catching mice. in england there has been a mouser at the Prime Minister's residence for centuries. The most popular of these cats was a stray name Humphrey who, in 1989, was first cat to receive the official title "Cheif Mouser to the cabinet Official." his care cost 100 pounds and was paid from the department's budget. It was said to be far cheaper than the fees previously paid exterminators. Humphrey, who was beloved in the UK, retired in 1997. The current mouser, name Sybil, took office in September of 2007.