Which 2013 film was Pixar's first prequel?

Which 2013 film was Pixar's first prequel? Monsters University was Pixar's first prequel.

Released in 2013, Monsters University travels back to the college years of Mike Wazowski and James P. “Sulley” Sullivan, before they become the professional scarers seen in Monsters, Inc. The story follows their rivalry, their academic struggles, and the Scare Games that eventually help turn them into friends and partners.

The project was initially developed as a sequel to Monsters, Inc. Pixar later changed direction and announced in 2011 that the film would be a prequel. Dan Scanlon directed it, with Billy Crystal and John Goodman returning as the voices of Mike and Sulley. Steve Buscemi also returned as Randall, while Helen Mirren joined the cast as Dean Hardscrabble.

Monsters University premiered in June 2013 and opened theatrically in the United States on June 21. It was accompanied by the Pixar short The Blue Umbrella. A common mix-up is to treat it as a sequel simply because it was released after Monsters, Inc.; chronologically, its events occur years earlier and explain how Mike and Sulley first formed their partnership.