His name is Fungus. He's an accomplice of Randall in his evil scheme. When Mike is rescued from the machine, they throw Fungus into it and when Randall (or Waternoose) comes back out, he finds him strapped to the seat getting his "screams" sucked out of his mouth.

EDIT: A link to his picture is included under the voice cast list at this Wikipedia link http://en.wikipedia.org/wiki/Monsters,_Inc.