The Once-ler is a main character in The Lorax, written by the popular children's author Dr. Seuss. The story of The Lorax is heavily environmentalist and warns against the dangers of corporations which exploit natural resources to satisfy their own greed. In the story, the Once-ler comes to a beautiful place with abundant wildlife and the fictional Truffula Trees. He cuts down the Truffula Trees and uses their colorful, fluffy tops to make the mysterious, does-it-all commodity called a thneed. The Once-ler is successful in selling his thneeds, but this leads to him cutting down all of the Truffula Trees, depriving many of the animals of their source of food and shelter. All of the animals leave because they now have no food or shelter and their surroundings become dirty. The Once-ler's thneed industry depletes the natural resources of the area and disrupts the life-cycles which once thrived there.

Eventually, the Once-ler feels guilty for his depletion of the environment and lives as a hermit, shut up in his home in the now wasted lands. The Once-ler's industry and exploitation of nature resulted in the near total destruction of his surroundings, but towards the end of the story we see him taking responsibility for this and trying to make amends. He entrusts the very last Truffula seed to an unnamed character (who we met in the beginning of the book) and encourages him to plant it and care for the tree.