The animated film whose Once-ler helped originate the Tumblr Sexyman concept around 2012 was The Lorax.
The Once-ler is the central figure in the 2012 animated adaptation of Dr. Seuss’s The Lorax. After the film’s release, the character developed an unexpectedly large Tumblr fandom, including dedicated blogs, fan art, fan fiction, cosplay, and shipping. That burst of online attention helped establish the pattern later called a Tumblr Sexyman: a fictional character who becomes widely treated as an attractive or compelling sex symbol within a fandom.
The Once-ler’s popularity also produced alternate versions of the character and a phenomenon nicknamed “Oncest,” in which fans shipped different interpretations of him with one another. The label is informal and its boundaries are debated, but the Once-ler is widely cited as the first major example. Later figures such as Sans from Undertale and Reigen from Mob Psycho 100 became prominent contenders, but neither originated the concept.