The Once-ler is the deuteragonist of Illumination's 3rd feature film The Lorax, the film adaptation of Dr. Seuss's book.
Before turning into a regretful recluse, he was an enigmatic business magnate who sought to expand his Thneed empire, against the Lorax’s grave protests. His thirst for industry ultimately causes the extinction of the Truffula Trees and the destruction of the environment, causing him to lose his factory and enter self-exile to atone for his mistake.