The name "Mutant Enemy" is taken from the song "And You and I" by progressive rock band Yes, of whom Whedon is a professed 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 monster cartoon figure crossing the screen from right to left and saying, "Grr. Argh." It was drawn and voiced by Whedon himself. In certain episodes of Buffy the animation was changed:
"Storyteller": The monster (overdubbed by cast member Adam Busch) 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 "And a monster. Sort of a Sunnydale souvenir, we thought. Grr. Argh." 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! Grr! Argh! He'll love you to <obscured> 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 "Grr... Argh..." 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."