Agent M is a monkey agent of the O.W.C.A. who first appeared on the Phineas and Ferb Musical Cliptastic Countdown episode. In this episode, she is seen typing the cue cards for the show on a typewriter. This is a play on the saying: If you had an infinite number of monkeys, and an infinite number of typewriters, they would eventually write the works of Shakespeare.
Despite that she's a monkey, the absence of her tail technically makes her a chimpanzee, which classifies her as an ape rather than a monkey, however, this may have been an animation error since she is shown with a tail in all other episodes.