The Once-ler is a fictional character from Dr. Seuss's book, "The Lorax." He is depicted as a businessman who initially appears to be ambitious and hardworking. However, his desire for profit leads to environmental destruction, as he cuts down all the Truffula trees to produce Thneeds, a product that becomes popular. As the story progresses, the Once-ler realizes the consequences of his actions, including the loss of wildlife and the degradation of the environment. The Lorax, a guardian of the forest, speaks for the trees and warns the Once-ler about the impact of his greed. Ultimately, the Once-ler's journey serves as a cautionary tale about sustainability and responsibility.