Mutant Enemy Productions is the production company created in 1997 by Joss Whedon to produce Buffy the Vampire Slayer. The company also produced the Buffy spin-off, Angel, and his two short-lived science fiction series, the space western Firefly and his high-concept Dollhouse, produced by 20th Century Fox Television and Mutant Enemy. Mutant Enemy also produced Dr. Horrible's Sing-Along Blog.

ts offices (made out of glass bricks) were on the lot of 20th Century Fox in Los Angeles, previously the home of Chris Carter's Ten Thirteen Productions. According to March 2006's issue of UK magazine The Word, the offices were closed not long after Angel was cancelled.

The following lists the production credits of Mutant Enemy
Buffy the Vampire Slayer (TV Series) 1997-2003
Angel (TV Series) 1999-2004
Firefly (TV Series) 2002-2003
Dr Horrible's Sing-Along Blog (Web series) 2008
Dr Horrible's Sing-Along Blob (Soundtrack) 2008
Dollhouse (TV Series) 2009-2010

Name and Logo
The name "Mutant Enemy" is taken from the song "And You and I" by progressive rock band Yes, of whom Whedon is a confessed fan. (On the special features of the Buffy DVDs, Whedon also says in an interview that he called his typewriter "mutant enemy".) The company's end-of-credits logo and mascot is an intentionally poorly animated vampire cartoon figure crossing the screen from right to left and saying, "Grrr. Argghh." It was drawn and voiced by Whedon himself. In certain episodes of Buffy the animation was changed:
"Becoming, Part Two" He says "Oooh, I need a hug" instead.
"Amends" He wears a Santa hat.
"Graduation Day, Part Two" He wears a graduation cap the image used is a reversed logo from the U.S. national college honor society Mortar Board.
"Once More, with Feeling" He sings the line in falsetto.
"Storyteller" He sings a line from the episode, "We are as Gods".
"Chosen" He turns and pulls a face at the viewer.
The episode "Bargaining, Part One" references the end-of-credits logo. Tara gives Giles a small rubber monster and says "Grrr. Argghh." In season seven of Buffy, there is a species of ancient Vampires called "Turok-Han" or Ubervamps. They look like more detailed versions of the Mutant Enemy.
In the canonical Buffy the Vampire Slayer: Season 8 comic books, issue 22 "Swell", on page 10, the slogan for an in-comic fictional product is "Vampy Cat! Grrr! Argh! He'll love you to the Santorio Corporation!"

At the end of the credits of the animated sketch comedy parody show Robot Chicken, a parody of the Mutant Enemy, Inc. logo appeared with Joss Whedon (guest starring as himself) providing the "Grrr... Arrgh..." As the enemy mutant rips apart the city, the scene cuts to an office where Joss Whedon is playing with dolls and an executive says to him, "Come on Joss, that's why you got kicked off Wonder Woman."

Jeffrey Bell
Elizabeth Craft
Steven S. DeKnight
Ben Edlund
Jane Espenson
Sarah Fain
David Fury
Drew Goddard
Drew Greenberg
Rebecca Rand Kirshner
Tim Minear
Marti Noxon
Doug Petrie
Mere Smith
Maurissa Tancharoen
Jed Whedon
Joss Whedon
Zack Whedon

Information from Wikipedia