The Lorax fandom has created many differnet AU versions of the Once-ler in a selfcest relationship, most of them showing up in fanart or fanfiction. As with other fandoms, these incarnations, who are often thought of to be different charcaters, are mixed and matched into pairings that are all various forms of Oncest. The oldest and most common form of Oncest, however (known as "Classic Oncest" by fans), is between the incarnations of the Once-ler shown in the movie. Though not actually two different characters, fans like to imagine that they are two differnet people, (or perhaps they are the product of time travel or a supernatural explanation) so that the pairing will work.
Young Once-ler is the form of the Once-ler who is seen most often in the Lorax film. He is shown to be an ambitous young man with dreams of fame and fortune, and he is quirky and rather kind, despite being blinded by short-sighted ambition. Though he tends to have a rather sarcastic nature in the film, in the terms of Oncest is is often interpreted to be more innocent and naive, especially when placed in comparison to Greed-ler.
When the Once-ler falls into greed and is corrupted by his fame and fortune, fans give him the nickname "Greed-ler," iconic for his green outfit and dishonest attitude. This form of the Once-ler is not seen very much in the film, as his only appearances are during a musical montage and a few brief minutes after, when he yells at the Lorax before the last tree falls over. As such, his personality is often left up to interpretation by the fans. Some of them interpret him to be a normal businessman, albeit still retaining his cocky personality seen earlier in the movie. Others see him as more monster-like, a power-hungry evil entity obsessed with making money and getting his way (this is often shown through his monster-like appearance in fanart). In terms of Oncest, he is often seen as a monster born from the Once-ler's subconcious, or in other cases, may be his future self coming to take over his life. The relationship between Once-ler and Greed-ler in seen to be rather complex, seemingly abusive from the outside, but loving and caring from the inside. Fans of the pairing believe that Oncie is the only thing that Greed-ler cares about more than money and is also the only one who can bring out his softer, caring side.
Old Once-ler is hiding in the shadows of his Lerkim during most of the Lorax film, only showing his face at the very end. He is much wiser and more melancholy than he was at his younger age, though he still has a very sarcastic and sharp-tongued personality. Once-ler fans seem to like the Old Once-ler a lot, but they seem to prefer his younger version, as they often write fanfiction about him magically going back to his younger age (usually through the Lorax's powers). As such, the Old Once-ler is never really seen in terms of Oncest, unless, like the other two, his is split into a different entity and plays a different part in the story.
Note: Please add only ones that are part of the RotBTD fandom