From Homestar Runner Wiki
Monkey D is a strange fellow from Seattle with a monkey head who shows up on postcards, generally in Easter eggs, and wears a white tanktop on which varying phrases are printed. First mentioned in the Strong Bad Email vacation, Monkey D was used as an example of the annoying crap-tastic emails Strong Bad gets.
In crazy cartoon, Strong Bad received an email signed by "monkeydude." It is not entirely clear if monkeydude and Monkey D are supposed to be the same person, although the MonkeyDude playing card in pom pom had Monkey D's picture on it.
 Screenshot Appearances