How would a reverse au situation with the Lorax and once-ler would work if it even could be a thing, like Lorax being a human and once-ler some sort of embodiment being (makes me think a bit about hexxus, but I guess once-ler would be more like the instinct of greed in people) your "not a sequel" post made me start to think about it but in a full total reverse deal
Toothy black ghosts are cool, but I already have funera.
He'd definitely be some greed-feeding trickster demon. Growth is one of human Once-ler's most defining features, so I'd give him a malleable physical form. Unaffected, he resembles a "human child" wearing a hooded onesie that looks much like the original Once-ler's weird bodysuit thing. When surrounded by selfish asses long enough, he bursts out of his attire and becomes this tall, black, maned beast. At his largest, the fabric only covers up his limbs, but it all grows back again if the transformation were to revert.
Storywise, this is a challenge. There are many story beats and personality traits to keep in mind and translate. In any case, the plot would have to revolve around a personal struggle of The Lorax.
The only setting I can come up with is that Lorax was born in Truffula, spent many years by himself, then strangers randomly show up with caravans and barbecues. More keep coming and he's so angered by the behaviour of these tourists, it attracts The Once-ler to his location -either spawned by the inconsiderate visitors or whatever thoughts are going through Lorax' mind. Is he protecting Truffula, or calling dibs on a piece of land that's not even officially his? Do his feelings touch "greed"? Doesn't matter, our boy is here to ruin all of our lives.
Upon meeting each other, he thinks Once-ler is a lost child. He allows him inside his home for a short while and his hospitality is what makes Once-ler become attached to him. He claims to have always been ignored or lashed out to, though later shows off the ability to create objects and life forms through knitting and tells The Lorax he's his toy in the literal sense of the word. Enough to give a man an existential crisis, but it's taken as a joke. Once-ler lets him know pretty early on he's not human, since he won't stop asking about his parents. I don't want the story to be about him taking care of a disguised creature, this scene is just to convey The Lorax can set aside his dislike for city folk and care for a lost "child". The reveal is not met with a huge response. Lorax accepts the spiritual world and Truffula is an unusual place by itself, so why be dismissive of this demon thing strolling around. He does ask Once-ler many times to return to the Heavens -generously assuming he's not from da land down undah.
The tourists that treat Truffula as their playground help Once-ler grow strong, but so does Lorax' weak offence against his antics. Once-ler's greediness becomes more severe and increases his size as well. He takes from the land, the tourists, and Lorax personally, and always has an excuse ready for his thievery. This Eden snake would probably love to see Lorax consider violent solutions for his entertainment, but in any case, he's the rotten little voice in his ear that insists his rage stems from his unwillingness to share Truffula. He needs to play on his insecurities, because if Lorax feels guilty and doesn't push harder to chase people away, then Once-ler has more people to potentially feed off of. Being lazy or inconsiderate is not synonymous with "greed", after all, he needs specific kinds of people to discover Truffula. He knows its remarkable plant- and wildlife will draw some "zealous" people. It takes The Lorax some time to accept human expansion is inevitable. The only right he really has is to try and educate visitors, he can't begin to threaten or chase them away. This mere man has less authority than the original Lorax, though many animals respect him for obvious reasons. Maybe the police keeps getting involved when he pushes his luck, but when he contacts them, nothing is done. The Lorax gets no support and basically everyone messes with him, so the only thing he can do for Truffula is clean up after everyone. His home is all he has in life. The work and stress become too much for this middle aged man. When his death appears inevitable, Once-ler's emotional abuse of the one person that cared for him stops being funny. Once-ler's late act of redemption here is to lead people to Lorax' body and expose them to the consequence of their behaviour. They don't question whether they played a part in it, as they all know him as a notorious nag and laughing stock, stupid enough to pick up trash all day and play doctor to even the most insignificant insect that happened to get a picnic basket dropped onto itself. Some people enjoyed leaving behind unnecessary work for The Lorax, admittedly because The Once-ler enticed them to. The game is fun when the target is just some tree-hugging Karen. Once-ler only manages to exchange a few words with the bedridden Lorax before he passes away. The political world gets involved and strict rules are implemented -or rules at all- to those who want to make use of Truffula. It becoming a regulated human establishment is not quite a complete victory for The Lorax, but well, the original Truffula couldn't get rid of its one city/town, either. Anyway, the end, this is mostly how it'd go.
