At Monsters Inc., Fungus was assigned to become Randall's assisant, Despite being partnered with Randall, Fungus doesn't seem to like Randall due to the latter's constant abuse towards him, though Fungus is too cowardly to speak out on this.
It later turns out that Fungus was involved of a plot engineered by both Randall and Monsters Inc.'s CEO Henry J. Waternoose III to kidnap several human children and use a harmful machine called that Scream Extractor (which would suck the screams out of children) that will render scaring tactics useless, as the company is facing an energy crisis. Despite his involvement, Fungus is shown to be quite uncomfortable with this as the machine was previously used on him, which almost drained his energy out, even feeling guilty when Randall orders him to activate the machine on a little girl named Boo.
However, two of Monsters Inc.'s employees Mike and Sulley, who had befriended Boo, manage to stop this by sending Randall to the human world and having Waternoose arrested.
It is unknown what happened to Fungus afterwards, but it turns out that Sulley (as the new CEO) has pardoned Fungus for his involvement and gave him (and the rest of the monsters) the job of making children laugh, which manages to help the company avert the crisis for good as laughing was discovered to be more powerful than screams. As such, Fungus is finally happy that he's free from Randall's control as he helps make children laugh to collect enough energy.
Fungus doesn't stand up for himself very often, mainly because he is scared of Randall due to the latter's short, explosive temper. He is much pleased to be saved from such wicked leaders as Waternoose & Randall, and became a comedian at the laughter-changed Monsters, Inc.
Fungus is a red monster with 3 brown eyes and chicken-like legs. He wears glasses with 3 lenses.
Fungus with Randall
Fungus after discovering that Randall captured Mike instead of Boo
Fungus and Randall about to use The Scream Extractor on Mike
Fungus after the Scream Extractor is used on him (after Sulley replaces Mike with him)
Fungus, Randall and Waternoose after Sulley destroys the Scream Extractor and saves Boo
Fungus doesn't reappear in Monsters, Inc.'s 2013 prequel Monsters University. This means that he must have attended Fear Tech or another monster college and met Randall after becoming employed at Monsters, Inc.
Fungus was originally going to be called Frungus.
Fungus isn't evil, he only serves Randall cause he is scared of him.