John Ratzenberger as The Abominable Snowman.The Yeti (a.k.a. the Abominable Snowman) was a monster who worked at Monsters, Inc. until he got banished (for an unknown reason) to the Himalayas. Despite being banished, he likes living in the human world.
When Mike Wazowski and Sulley get banished to the wasteland in the Himalayas, the Yeti greets them and makes them feel comfortable in his cave. He also shows himself to be a comedian, such as when he mentions about his friend Bigfoot, who has called himself "King Itchy." When he mentions something about a local village, Sulley, still wanting to help the human child he has accidentally brought into the Monster World, suddenly demands the Yeti to tell if there are any kids in that village so that he will be able to sneak back into the Monster World. However, the Yeti warns Sulley never to go out in a blizzard, but Sulley does so anyway.
Frank Oz as fungus. Fungus is Randall's assistant at Monsters, Inc. He is a small red monster with three eyes and legs, similar to those of a chicken. He wears three lensed glasses. Fungus is rather wimpish, due to the wrathful temper of Randall, and helps him with his plot to revolutionize the scaring industry, although he regrets it. Fungus is initially a kind person and means well, even apologizing to Mike about his capture. After Randall is banished to the human world, Fungus is a lot happier and takes on the job of making children laugh.
Sam Black is the voice of George.
George is a scarer on Scarefloor F at Monsters, Inc. He is a furry monster with a horn on top of his head. George is also very good friends with Mike, Sulley, and Claws Ward.
When he comes out of his door one day, he has a child's sock on his back and when his assistant, Charlie sees the sock on his back, he gasps and shouts, "23-19! We have a 23-19!". The CDA agents arrive and remove the sock from George's back. The agents then shave George's fur off, have him showered, and place a cone on his head.
Later on in the film, Sulley puts some of Boo's toys in George's locker which George accidentally left open, and when George opens it the next day, the toys fall on top of his head and Charlie calls again for the CDA agents who come and jump on George again putting more cones on him.
Later, when Charlie gets George a door from Nepal, Sulley bursts through after being banished and a sock falls on George's chest. Just as Charlie is about to call for the CDA again, George stuffs the sock into Charlie's mouth and throws Charlie into the kid's door and walks away happily while whistling.
At the end of the film, George's fur grows back and works on his own on the new Laugh Floor making kids laugh instead of trying to scare them.
Daniel Gerson as Needleman / Smitty.They work on the scare floor as a general maintenance and errand monsters at Monsters, Inc.
Needleman, a thin brownish monster, and Smitty, his short greenish friend, worship the ground that Sulley walks on. They really support Sulley in his race to beat the all time scare record and have a lot of respect for all the scarers on the floor. They shred the door that Claws Ward has used when a kid he had attempted to scare turned out to be unafraid of him. They also take out the garbage almost nearly destroying Boo, who manages to get out before they put the garbage in the crushing machine.
In one the film outtakes, Smitty mispronounces Sullivan's name as "Solomon" and Needleman tries to tell him that his name is "Sullivan," but the two begin quarreling as they end up ruining the scene and making matters worse. Another outtake shows the duo about to shred the door that Claws Ward has used; the door shredder chews the door, but then takes the two on a wild ride across the scare floor and knocks over the crew equipment.
Steve Susskind as Floor Manager (Jerry).Jerry is the floor manager for Monsters, Inc. He is a supervisor of Scare Floor F, where Mike and Sulley work.
He first appears in a commercial for Monsters, Inc. that Mike and Sulley watch in their apartment room.
He counts down the amount of time left in seconds until it is time for the monsters to go into the human world to scare the children to harvest energy.
When George Sanderson exits a child's room with a sock attached to his back, Charlie alerts to Jerry, who rushes over to activate the alarm that summons the CDA. He also has seven fingers on each hand.
Bonnie Hunt as Flint. Flint (identified on the Disney website as Ms. Flint) supervises the training of new scarers. She is weary of the task, especially when dealing with the likes of Bile. An example is when she asks Bile what he did wrong after coming in. She rewinds the footage to reveal that he left the door wide open (literally). She then asks him why is this bad, to which Bile replies, "It could let in a draft?" Mr. Waternoose cuts in and says, "It could let in a child." He then explains how toxic kids are, and for one to come in contact with any monster is dangerous because a single touch could kill them.
Jeff Pidgeon as Bile. When first shown at the very beginning of the film, Thaddeus struggles with his test simulations, leaving a door open and utterly botching his attempt to frighten the mechanical child. He is accident-prone and a bit of a coward.
He is seen sitting at in front of the Sushi Chef at Harryhausen's on the night Mike Wazowski takes his girlfriend Celia Mae to the restaurant for her birthday dinner.
At the end of the film, he is seen returning to the Laugh Floor from a child's room, and he seems to have successfully made that child laugh.
He also appears in Mike Wazowski's company play Put That Thing Back Where It Came From Or So Help Me, portraying the Sushi Chef during the restaurant scene, as well as singing with the rest of the cast at the play's finale.