There is no one single answer to this question, as the creators of Dr. Horrible’s Sing-Along Blog remain a mystery. However, some possible suspects include: Neil Patrick Harris, who starred in the web series that inspired the show; Joss Whedon, the creator and writer of the show; and Zach Galifianakis, the actor who played Dr. Horrible.
Regardless of who created Dr. Horrible’s Sing-Along Blog, it is an entertaining and clever show that deserves to be more widely known.