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 was 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 a tail technically makes him a chimpanzee, which classifies him as an ape and not a monkey, however, this may have been an animation error since he had been shown with a tail in all other episodes.