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, he 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 he's a monkey, the absence of his tail technically makes him a chimpanzee, which classifies him as an ape rather than a monkey, however, this may have been a graphical error since in "Y.M.C.A. (Phineas and Ferb Remix)" he is shown with a tail.